i try to write down my other ideas (but am really really bad at it, trying hard to do better) but I'm usually writing a lot of other short stories, poetry and stuff for submission open calls and competitions all the time while working consistently on my main project (atm, "Damsel in the Red Dress")
I was working on "Hushabye Prince" at the same time, but that is not the case now. I think reasonably I could probably do about two novels at the same time, and my goal is to start working toward another one reasonably soon lol, I have a whole bunch more novel and story ideas but I don't know which to do next lol.