I like to take a few weeks between writing a novel and editing it, during which time I either write another novel or edit another novel. This has resulted in my having four not-quite-finished novels on the table right now. I refuse to write another until I have hammered the nails into the first, so four would be my limit. That's my first 'proper' novel which needed umpteen edits, but is going through betas at the moment. The others are in states of: needs betas, needs a second edit, and needs a first edit. I'm usually a week per edit when I'm focused, so I anticipate getting back into writing near the end of July. Maybe by then I'll have two novels done even (or at least in beta edits). That would be a relief.
As a kid, I read multiple books at once. I don't know why, I just did. Usually three at once, it just wasn't a problem, and that seems to translate to writing. Three of the novels I'm working on tie into each other, to the point of having PoV characters in common. Makes it easy to keep the world straight, the characters straight, and because I'm right in the middle of things it's easy for me to pluck plot elements from other stories and explode them later on. So that actually works really well. The other book is a random children's novel that has nothing to do with the others. Literally, nothing. To me, it's like watching a TV series and reading a few books in the meantime and going out and seeing a movie. They're separate activities, so they stay separate in my mind, no overlap.