I assumed this would be a fairly straightforward question to answer, but it turns out I was wrong. The short answer is, under optimal conditions, you can obtain a maximum of 369 Pokémon in just White without trading, transferring, or otherwise communicating with other games. As for the long answer, here's how I arrived at that number:
According to a quick count, of the 649 Pokémon in gen V, 180 are completely unobtainable in-game, 6 are event-only or require another event Pokémon to unlock, 18 are exclusive to Black, 1 requires a Black exclusive to unlock, 8 can't be obtained if another Pokémon in their group is chosen, and 15 are trade evolutions that aren't found in the wild in White, leaving 418 Pokémon obtainable in White alone.
However, this total includes the 97 Pokémon that can be obtained from White Forest or evolved from Pokémon found there, and this is where things get messy. White Forest can only have 10 NPCs visting at any given time, and inviting additional visitors requires communication with Black. Each visitor unlocks a unique Pokémon in the grass (all of which are part of three-stage lines), as well as one of four Pokémon in the water that is shared with other visitors.
The optimal combination of 10 visitors to White Forest includes all three NPCs who unlock Pokémon with split evos, one of the NPCs that unlocks Nidoran (which can then breed to produce the other gender), and any six NPCs that unlock Pokémon other than Lotad in the grass (which other NPCs can unlock in the water) or Pokémon that require trading to evolve, provided that said combination also covers all four Pokémon found in the water. Assuming this optimal combination (which gives access to 46 of the 97 Pokémon in White Forest), and omitting the seven trade evos which were already excluded above, that leaves 44 further unobtainable Pokémon, for a total of 369 if my math is correct (it probably isn't, someone double check).