I'm a one book at a time kind of person. I usually BUY multiple at once and then choose which order to read them, though.

Same just reading one even though 1 I ordered has just arrived with 2 more on the way lol

Depend of what im reading, if for exemple, im reading exoteric books i usually read even 3/4 at the same time to get the reference or discover something more in depth, especially with alchemy, goetia and qabbalah.. even with musical teory book such orchestration historic or contrappunto's tecnical book, is veryy common that you find me even with 4 books at the same time... but when i want to read a librogame like lonewolf or a romance like the ancestral story, i can read maximum of 2, depend if i didn't like so much.. if i start to fall in love, i cannot read anything else until the end.

At various stages of reading, a few, actually readying, only 1 book at a time, at most 2 if one is fiction and one is non fiction.

One, to fully dedicate CPU and RAM to the story.

I've got a fiction (Priory of the Orange Tree) and non-fiction (Failed State) on the go at the moment, which is fairly typical, and sometimes add a third if audiobooks count. It's common for one of the two/three to grip me much more than the others so I may as well be focusing just on one at a time de facto (if not de jure).