I read 1 book at a time. I used to only read in a single read. As in, I started the book and read til I finished. Most books only take a few hours, so I'd make sure I had three or four hours free and read til it finished. LOTR was hard, it took a weekend when I was a pre-teen. Nowadays, with kindle on my phone, I read everywhere. I read in queues. I read walking to the shop. I read on the bus. I still only read 1 book at a time though, because it only takes about 3 hours so whats the point of switching?
I do something similar I have one book I focus on and usually read about half then i will either keep reading or read another one so i technically do read multiple at a time although I focus on one for a while rather then constantly switching(with audiobooks and not in one session)
Before E-readers were a thing, I used to read 3-4 books at a time. But it was dependent on location, like I'd read one in bed, one at the coffee table, one for while I was holding court on the porcelain throne, etc. Now that I can carry thousands of books in my pocket (seriously, kindle and iBooks are taking up over 1gb of space on my phone each - that's a LOT of books), I'll read one at a time until it's done.