I learned as a defence mechanism against all the pins that I would throw into the air just for fun. After being hit a few times on the noggin, you start to think being able to catch them is a good idea.
Also have a neat (ball) pattern that involves rapidly crossing arms–to use a hand to catch the ball on the opposite side–and uncrossing.
I can juggle balls, clubs and torches. No more than 3 of any at a time; just the standard way of throwing them, nothing fancy.
I am a self-taught juggler of 3 objects at a time, usually balled up socks
I can juggle three objects, but only in the plain vanilla pattern.
The hardest aspect of learning for me was breaking the tendancy to throw the objects forward - I would wind up chasing them around the room.
I often hear people comment about how difficult it is to juggle objects that aren’t all about the same weight. I’ve never found this to be a problem for me.