Numbers juggling, or simply trying to juggle as many as you can, is arguably the most noble and glorious of all the juggling styles. It can also be one of the hardest. In the time it takes to learn just 5 clubs or 7 balls you could be really good at 3 balls or 3 clubs and in the same time master a large number of tricks instead of just the one number.
From the performers point of view (Russian circus stars excluded) it's not a very good use of practice time. You usually get very little performance time out of numbers juggling and the audience would rather you eat an apple any ways. Finding a stage with enough ceiling height can be tough and outdoors wind can be a huge problem. Also with numbers you tend to look up more making stage lights or the sun into a bigger problem and looking up is no good for your character development.
But for us purists numbers rock, so here's a few tips to help on your path to greatness and a literal game of one upsmanship.
Sounds obvious but it often isn't. It's not uncommon to find that the reason you can't get past 20-50 or so throws is you're not breathing. 5 clubs is a common place for this to happen but any time you're going past your limits and pushing yourself you can end up holding your breath without realizing it.
Speed is so important that spending a bit of time consciously trying to slow down often adds a few more tosses to your pattern.
Rhythm helps any juggling pattern and it makes them more pleasing to watch, but in numbers it has to be right or it falls apart. If you throw one ball 1/10 of a second late and the next 1/10 early the rhythm is off by 2/10's and your pattern is hosed. Listen to your pattern and get used to using what you hear in addition to what you see to make your patterns smoother and easier.
And it's not just the odd toss out of rhythm either, listen to your hands, do you hear a nice even alternating catch or is it an uneven catch catch - catch catch - catch catch kind of thing.
Pay attention to your starts, it's common to rush these with the hope of settling into a rhythm once the pattern is up. This can seem easier when you're just learning a number but it's a setback as you're learning to start yourself in a pattern that needs correcting.
Since you're often making much higher throws in numbers juggling than when doing tricks with 3 small errors in your aim are multiplied into catches farther away from where they should be. In all juggling if the throw is accurate the catch is easy. Not much different in numbers than any other juggling.
Try to feel where your hands are catching. Are they at the same height? Are they all over the place? Is one in front of the other? Fix these problems as you find them.
If you have problems balancing your pattern and have difficulty getting one hand to throw higher for example, flip the problem around and try to have the other hand throw lower. Sometimes these simple psychological tricks work wonders.
Odds or evens?
Somewhere around 5 to 7 objects most jugglers will find they have a preference for odd or even numbers for each particular prop. In the most basic patterns odd numbers cross (The Cascade) and even numbers juggle half in each hand without crossing (The Fountain). Most people prefer odd numbers for balls and clubs and even numbers for rings. This isn't a hard and fast rule by any means and you'll find many examples to the contrary.
There are difficulties with thicker props like balls and clubs in the fountain pattern since they pass closer to each other and a prone to collisions. Rings work great in the fountain since they are so thin you can often get away with one going on the wrong side of another and correct from that easily. Rings work pretty well in a cascade too so the preference for odd numbers in rings is rather minimal.
Perfect the lower numbers
All your skill in the lower numbers will carry over into the big stuff. Working on 7 balls? Spend a lot of time with 5 & 6 making sure the pattern is as perfect as possible. Also try for endurance with your lower numbers, it's good to keep track of your longest times and try to break your records regularly. Once you can get a few minutes with one number move up to a higher one. You may not get too much benefit from a 3 hour 3 ball run.
You can't juggle numbers without accurate high throws. Dropping down a number or two from what you're working on and trying to throw higher and slower is very helpful for learning numbers. Juggling 4 balls at a 6 ball height or lofty 3 club double spins to work your way up to 5 clubs will let you slow down and get longer runs with the height you need. This exercise is good for those who have their hands all over the place and need to work on placement.
One hand starts
Starting with 3 balls in your right hand (or 3 clubs, or 5 rings, whatever), one at a time toss them to the left and catch them and position them to throw them all from left to right. At first you can do this nice and slow, toss one, catch it, then toss the second. It can take a while to find the grip and release that works right for you with any given prop. Pick a number that's half or more of the number you're working on, so for 7 ring patterns work with 4 rings, 6 ball pattern use 3 balls. This can be done super slow and will build skills for your launch and the final catch.
This is the same thing as the one hand starts above but you bring the speed up to what you would use in a pattern, and you don't catch them but keep them juggling. This is often used for 5 balls & 5 clubs using just 3 objects like in this animation. Cutting out those 2 objects simplifies things and gives you a lot of room for corrections while getting some time making the correct throws for a higher number. This can be tricky and once you can do a snake for a certain pattern you're very close to doing the full pattern.
Flashing (one and only one toss with each object followed by catches) is often the only practice technique many jugglers will use, flashing has it's place but it can breed bad habits too. You have to watch out for rushing the objects out, throwing the last ones extra high to make the catch easy, funky rhythms, hands going wider and wider and other issues. A flash will let you get away with a lot of things that will stop a pattern so practice with it but watch out for cheating just to make the catches.
Pay close attention that all tosses in a flash are the same height and the rhythm is smooth and consistent. Once you can pull off a near perfect flash you should be getting long runs.
Dominant hand leading trick
This applies to just the even numbers where your hands are synched, throwing at the same time.
Say you're working on 6 balls and you can do 3 in your right hand pretty well but can't get any good runs with your left. Try going straight to 6. Thanks to the wonders of the human brain the weaker hand will tend to mimic the stronger hand and you may find you can run 6 longer than you can juggle 3 in your weak hand.
Are you relaxed? Leaning forward or back? Shoulders scrunched up?
Arm position is important, you don't want your arm up or in front of you you want your upper arm hanging relaxed and down with your elbows bringing your hands in front of you. Catches should be made in front of and close to your stomach instead of out in front of your chest.
Have you got a video camera? It's a great tool for learning juggling. Set it on a tripod or a chair and tape yourself from the front, back and sides. Look for one hand throwing higher than the other or in front of the other and things like that. Spend some time trying to bring up the weak side of your pattern, if you're right handed try starting with your left sometimes.
Try to find the most efficient pattern you can, it takes less energy to juggler perfectly and of course consistency and endurance go up when you use less energy. Simply saying use less energy isn't that helpful but if you try to relax and think about it consciously as a regular part of your practice it can help you juggle more for longer. And that's when the money and the chicks start rolling in :)