I think its currently 15 (per faction)?
I think bumping it up to 20-25 will significantly enhance the experience. Of course there would probably have to be another helicopter available.
The main problem with PVP (that ive noticed at least) is that the distribution of players is too scarce. This is why red circles were needed, since otherwise finding PVP was too rare for people who enjoy it.
With a higher player capacity, red circles could be reworked or even completely removed, as natural PVP would occur significantly more often. PVP systems should be developed around naturally occurring encounters, but they are simply too rare atm, with probably only about 15-20 people per server actively looking for PVP (if you are lucky).
Personally I think if there is a future for PVP in this game, it involves more players per server. More natural encounters, more danger, more fun.