Juggling with obscene numbers of balls. Generally, numbers jugglers are the members of the local juggling club who are at the same time the most admired, and the most hated. Admired, because, well, WOW. Hated, because you know that you're not going to get anywhere near that number of balls even if you juggle until your arms fall off and your head explodes with frustration.
(Don't worry, I don't *really* hate you. But stop doing the blatantly impossible, dammit!) 8^)

Numbers juggling is generally defined as anything above five balls. If you're a club juggler, however, you have a harder job, since you're only considered a numbers juggler at or after five clubs... a task immensely harder than five balls.