I tend to like to keep between two and five RPs at a time.
One, being disabled, I have a lot of time on my hands whereas others do not have the same level of... luxury, I guess? This way I can keep busy between several people and it doesn't feel like people need to reply ASAP to keep my attention.
Two, I like to explore different themes in RP and not all of them can be covered in just one! Plus, sometimes you need some super lovey-dovey sapphic romance to balance out the darker plots with other people. Bit of Whee to balance the woo.
Thirdly, I just enjoy meeting new people and learning what makes them tick - especially if we have chemistry.