Yes, this is indeed the case — the more Altars you smash, the more ore your world will have. Smashing a Demon Altar actually does 3 different things, provided you've unlocked hardmode:

1. Adds a bit of the ore named in the broadcast message to your world.
2. Summons a few Wraiths to your location.
3. Has a chance to either corrupt or hallow a random part of your world by planting a single block of Pearlstone or Ebonstone.