You can play 6 500 events and they will also be counted as long as they are your 6 best results.
It's a tough ask to expect a top player to play 21 events these days and go deep in all of them though, which is probably why none of them do it.
The rules were changed last year - I think before that 4 x 500 and 2 x 250 were always counted, but I could be wrong. Now it is a bit more flexible, so long as the 4 x 500 rule is met.
Not surprisingly, Ferrer has probably come closest to the possible 6 x 500 points scenario. Looking at his rankings break down, he played 5 x 500s (including Monte Carlo), Olympics and DC. But his 6 best include 2 x 250 titles, and he has two more in his non - countables at the moment!http://www.atpworldtour.com/Tennis/Players/Top-Players/David-Ferrer.aspx?t=rb
Tipsy, on the other hand, has done so badly in the 500s that he only counts one (90 points), and MC (90 points), but he did play 4 so he can count more 250s in his 6 best.http://www.atpworldtour.com/Tennis/Players/Top-Players/Janko-Tipsarevic.aspx?t=rb
It seems Nadal has zeros for Valencia and Beijing which remain for a year, and he can't count his 6 best. So he has a few non - countable points although he played so few tournaments.
I hope that makes sense!
Davis Cup only earns points in the World Group and WG play offs.
I can't see that Andy has actually committed to any 500s yet, except Monte Carlo, which counts for some reason. But entries have to be in 6 weeks before, so there's still about 3 weeks to go before the Barcelona deadline.