At the current stage, 2 for most games, 3+ for a few. What matters is how efficient the cores actually are.
More cores allow you to run more things same time, not run faster games (as a general rule).
Note that when i say 2 cores i mean "just the game" skipping OS and other running programs. This is why 4 cores are mostly used now, 2 for a game 1 for windows, 1 idle/side programs.