At the time of writing this, I am actively reading four books.
It is not unusual for me to read more than one book at a time; in fact it's pretty much how I work. I tend to have a physical book and an ebook in progress at all times. This is convenient simply because the ereader is handier when I am out and about and the paper book I find easier to read when relaxing. The real downside is that it just takes longer to read each book.
I've done this since at least University (where I would be reading text books simultaneously with fiction) and my record is seven simultaneous books. I finished six of the books, the other was a re-read of Tehanu: The Last Book of Earthsea by Ursula K Le Guin which I didn't get to the end of (and the copy has long since wandered off to wherever unattended books go). I suppose technically I am still reading it but you have to draw the line somewhere and after nearly 25 years I think it's safe to say I'd need to start it again.
My wife doesn't understand how I can do this, she wonders how I don't get confused between the books. I suppose part of this is that the books are (usually) very different, or have a particularly identifiable style. And I have to admit I've not tried reading books which are close in story or style, or the same author. It might be an interesting idea one day to try it and see if I do get the books confused.
My reading list has doubled from it usual size at the moment simply because books come along that I have been looking forward to reading; when I get a new book I evaluate what I am currently reading and sometimes the new book ends up on top. The existing book is left - bookmark in place- until the new book is finished. Or as happened now, yet another book I have been even more impatient to read has come along so now the stack has grown larger. There are times when I have to avoid getting any new books because I know I will want to read them and I already have enough to read.
So now my worry is how to find enough time to finish my current books before the next 'must haves' come along to be read, digested and reviewed. There simply isn't enough time in the day...