I'm usually pretty good about not joining more than I can manage, to the point that even when I think I've gone and done it I've actually just reached the max instead. I guess it's an instinct at this point lmao.
That being said, the amount of RPs I can participate in has definitely changed and tends to fluctuate even now. Way back when, I could be in like ten different RPs (and running like 6-8 of them haha). These days I can probably run a similar number if I really want - I kind of am if we're counting TTRPGs as well. I DM two campaigns actively and routinely add mini's and one shots to that. I'm also now GMing two text RPs here on top of that. I couldn't really participate in more than I currently am though. Maybe I could squeeze one more.