10.
It seems like the vast majority of games I have ever seen or heard about don't generally get beyond level 10-ish in whatever edition, no matter how much they play.
I'd rather 5e reflect what most people actually do.
I voted 20, but I don't think 20 is going to be the same 20 as it was in previous editions. With the flattening of the power curve, 20 is the new 12 or so. At least that's the impression I get.
10 levels in basic set - 5 minimum, but I would prefer 10. More options for those + another 10 up to 20 in advanced (PHB). Epic levels in their own book.