I haven't really counted.
I am usually in the middle of a few.
There's my pbook(s) for Shabbos/Yom Tov, and my ebook(s) for the rest of the week. Those coincide quite well, as a rule.
When I discover a new author, I will
sometimes start them regardless of what I used to be reading.
New releases by old authors frequently push out whatever I am currently reading.
Sometimes I start a new book just because I am in the mood for a change of pace (though I try to find some other book I am in the middle of anyway
).
I don't have a lot of self-control in this regard -- fortunately, I usually keep track of events in different books pretty well.
I do read most of those books eventually, and the ones I don't finish probably weren't quite as enthralling.
Even those I sometimes get back to.
I would not be surprised
at all if I occasionally get to 14 books at once
without counting the ones that I have sort of abandoned.