I've just checked the export_descr_buildings, and here's what I saw :

-Priest : Agent limit 1 for the two first rank of building (small church/chapel, church) then the limit grows to two with abbey and higher.

-Merchants : Agent limit 1 whatever the rank of the building is(from grain exchange to huge market)

-Spy Assassins : No limits.

I think I'm right, but better ingame tests are needed.