DD is the book-- er, I mean, she's the one that juggles multiple books at once.
DH and I both tend to read them one at a time, but DD16 hasn't done that since she was about five, when she gobbled up beginning chapter books like potato chips (one after another as long as we'd let her, basically). She will do it now, but only if it's inconvenient to carry more than one, say on a car trip or vacation.
She seems to read as many as half a dozen or so at once, and perhaps more if she's reading below level.
Currently, for example, she's got Ulysses and Finnegan's Wake going alongside Invisible Man and a fond re-read of Watership Down for late-night. On top of her college reading load, I mean-- none of that is assigned.