So far as I can determine it’s two, but that’s only because WIP’s 3-12+ don’t get as much time as I have available. I do tend to shift around from one project to another, more often due to burnout than anything else, but also sometimes because I just get more enthused about something else. I can’t even imagine what would happen if I forced myself to write something that I’m not currently engaged with. So yeah, I hop around a bit. And also: same method within most of my stories, I just write whatever scene appeals the most and then stitch it all together later (a huge plus with planning IMO).
Right now the one I’m spending the most time on is easier to write than the other one, which is a longer story with more moving parts. So whenever I get bogged down with it I write shorter pieces (c70K). The shorter ones are actually easier to write (planning, not pantsing) and the closer I get to completion the easier it is to stay on point.
It works for me. I rationalise to myself that so long as I’m making forward progress with something I’m getting ahead of the game. If I never finished projects I’d probably question my methods, but since I do manage to crank out finished products (I also have stuff other than novels) I’m happy to stick with systems that work.
Short fiction pieces don't count as WIP’s, but I spend time on that too if the novels are too daunting. I’m currently writing for a short story competition, otherwise I’d be almost done with the first draft of my new novel. I’m not stressing over it, and the longer I stay away from a particular project the fresher I am when I get back to it and the easier the writing flows. And weirdly, I never lose track of where I am with any of my work, even after not opening a file for a few years.