Well, if you are Blizzard, it is a question with a short answer: 14. Out of the nine maps in the current pool, seven have exactly 14 bases, one has 12 and one 16 (our beloved Tempest, which happens to by my most favourite 2v2 map).
It may not be immediately clear what that means. 14 bases that is a lot, isn't it? No, it isn't - for four players. Consider for a moment the only two remaining 2player maps in the 1v1 pool (Scrap and XNC): both of them boast full 10 bases - that's 5 per player, in contrast with the 3.5 provided by the majority of 2v2 maps. The trend actually goes further (with the climax in the form of "Monsoon" with 14 bases for three players, but Blizzard has admitted not taking 3v3 and 4v4 seriously, so let's ignore them for the time being (however sad that is).
Many TLers say that 2v2 is a joke and should not be taken seriously, but I beg to differ. There is an amazing depth of possibilities in the number of matchups and unit combinations and the fact that you have two supply caps instead of one makes up for the lack of "epic armies" that many cite as a reason for SC2's superiority to BW. In my opinion, the perception that 2v2 is all cheese and rushes is perpertuated mainly due to the map pool.
It's not that the rush distances were too small - and with 3 vetoes you can't get rid of enough maps (Moonlyth, Red Stone and that abomination starting with G) so that you don't have to be too far from each other. The problerm I see is that on most maps, there are not enough expansion that could be reasonably secured. The whole idea, that there should be one natural for the whole team (which is obvious in some of the design) is just outrageous. Yes, the fifth on XNC is quite difficult to get and that's absolutely fine with me. But the "fifth" on a 2v2 map is actually my (or my ally's) third!
I actually did never count the bases on any map, until we (me and my wife) played several matches on Ruins of Tarsonis, where we mined out the entire map We play TvP and when the other team also has a Terran, it often comes to a stallmate in the middle, with two heavy siege lilnes on the immense cliffs below naturals and noone willing to attack. (Sadly, we lost all of these game, because we always mine out first and thus we just go, attack and eventually lose as we don't have income but they do.) Well, mining out is a plausible scenario, but doing so in 40 minutes feels, kinda, short. You get into a situation, when as soon as you hit the lategame, you have to actually start preparing for the fact that there soon won't be any resources on the map. It's not "something that happens sometimes", but a likely scenario.
Interestingly enough, as I started to write this blog, Blizzard unveiled two new 2v2 maps, with 15 and 16 bases. Could we see a trend here? Could we ever get a nice 20-base map?