So we know that Uxie and Regirock are in the game, so 99% chance that the rest of their trio members are. So... that makes how many legendaries?
Kyurem, Cobalion, Terrakion, Virizion, Reshiram/Zekrom, Latios/Latias, Cresselia, Regigigas, Uxie, Mespirit?, Azelf?, Regirock, Regice?, Registeel?
So if that's all of them (presumably), I counted 14 legendaries. Not to mention we don't know about those magma/water caves...