i realy dont think that there is a limit. i mean i had over 20 cards in the recruitment pool b4.
as for the restriction, i know its easy to restrict the mercs to a region: furthermore, restricting the mercs to a single faction shouldn't be hard. You would have to change their entry in the model.db and I believe there is a script for all mercs. this might be the mercs txt in the world/map/imperial_campaign that im thinking of...i have never edited this. Just an educated guess.

As for the hardcoded limit per settlement. if there is a limit its big, for i had 4 rows of recruitable units b4....and thats ALOT