Reshiram is the Pokemon Black version only pokemon that’s on the cover of the game.
There can be two opportunities to catch Reshiram in the game. The first time will be in N’s Castle after you beat the Elite 4, and you’re basically forced to catch it unless your PC boxes and your party is currently full to the point you literally can’t catch it.
If you didn’t catch it on your first meeting because your boxes were full, then you can see it again at Dragonspire Tower.
You will not encounter Reshiram again in Dragonspire Tower if you caught it in N’s Castle. Reshiram will always be level 50 when you encounter it.