I like to do one advanced (Precipice) and one casual RP at once. Occasionally, I might do another Advanced but never anymore than 3 RP's at once.

I usually stick to around 3 casual RPs at once. I might only be in 1 or 2 if I feel like I can't handle more.

Usually 2 actively in the IC, but up to 10 or more 'in the works' at a time.
And everything tends to wander towards casual.

How I decide when to start/end an RP is a bit more questionable.

Zero right now. When I'm able to RP, I normally try to stick with 1 or 2. Any more than 4 and I start to lose my marbles.

I am currently in 5, all Advanced-ish. All but one are 1x1.

I work 40 hours per week so I don't really have enough time to make more than one or two posts per day in each RP. Luckily, my group RP moves pretty slowly so some days I don't even have to post. :P

I never in less then 10.Most are free. I get easily bored.

I am currently in 4 active RPs.

Two of them I am the GM. One of them is faster with more people while the other is a slower with much less people.
Two I am just a part of. Both have a decent activity trough one is showing signs of a possible slowing or dying in the future - we will see.

I usually have around 4 RPs, trough on the Old Guild I had I think 7 or 8 active RPs at one point (it was summer and I had only share the PC with one of my family members so I had more posting time). But since I will be WoTing heavily this summer I doubt that I will go over 4-5 RPs this year.