[Sillium]

You can set "tf2_perm_crit_min_players" to the number you want (in your example 12). As soon as this number is reached a vote gets started. If there are already enough players there when the map starts, then the vote should start at map start.
It will not reset if players leave the server. The setting will stay till the next map starts.

This is untested. I just quickly edited the plugin.