I'm a book reader (unlike the vast majority of Americans who never buy or read books) and I've always got a few going at once. I split my reading pretty evenly between fiction and non-fiction, and two of each at a time is my normal minimum.
Sometimes, when I'm going through a stack of books (yes, I have stacks of them) considering a new read, I'll start the first chapter and then fruitlessly search for a bookmark. Anything will do in a pinch, and if I happened to be reading while sitting on the toilet-a practice I never engage in (the reading part, I mean)-I could even see myself using a square of toilet tissue to save my place. Most often though its some old scrap of discarded paper, or a bit of torn-off magazine page or something.
I hate the practice of bending the corner of a page down, and I never do that, so recently I bought some blank index cards to use for the purpose. I start more books than I finish, and by that I mean those first few pages. Once I get going, however, I don't usually stop reading a book midway. But those others are scattered about everywhere, waiting for me to pick them up again, and if the human lifespan is eventually extended by a few hundred years through a scientific/medical breakthrough, I'll get to them, too.