To get every single non-Mythical Pokemon, you would need a total of 7 games: every game in Gen 4, and both of Black 2 and White 2. Every non-Mythical pre-Gen 5 Pokemon can be found in at least one Gen 4 game, save for the Regi trio, which can be found in B2W2, along with every single non-Mythical Pokemon introduced in Gen 5.
However, if you want to get a majority of Pokemon, you would probably do just fine with just Platinum, either HG or SS, and either B2 or W2. You would miss out on about 40 Pokemon with this method, due to version exclusives and starters, but if you can get someone to get the other games and trade them to you, then that might be your best bet.
Hope this helped!