Yes, this is indeed the case. The more Altars you smash, the more hardmode ore your world will have. Note that every ore cycle yields less and less ore — fewer veins are generated each time, plus they have a chance to generate inside already-existing deposits.
Smashing a Demon Altar actually does 3 different things, provided you've unlocked hardmode:
- Adds a bit of the ore named in the broadcast message to your world.
- Summons a few Wraiths to your location.
- Has a chance to either corrupt or hallow a random part of your world by planting a single block of Pearlstone or Ebonstone/Crimstone.