/sci/ - Science & Math

I'm not sure if this question belongs here or /qtddtot/ but here it goes.

Asteroid belts as depicted in television where the asteroids are mere meters apart are not generally possible because the asteroids would've gathered together to form planets due to their gravity long ago. However, how dense an asteroid belt can be in the long term? What mechanisms would result in the formation of this densest belt and to maintain it?