It's unlikely you'll get into a group for the DoA with a Dervish. Most groups there use a tank/nuke/heal stratagy, and are often too ignorant to realise that a dervish can tank fine as well.
You'd have to be an obsidian tank if you wanted to stand a chance of finding a group. This page should help:
http://www.pvxwiki.com/wiki/Build

/E_Obsidian_Dervish
As for the number of Dervishes, yes, there are quite a few, and most of them are terrible.