You get a base 0.5% conversion everywhere from your Clan religion .
Monks convert at 1% per level plus and bonuses from retainers and skills {the total can get quite impressive .
Monistaries at top tier can cancel out a Namban ports' conversion {they're 1% per level as I remember} .
The maximum {all agents total} you can have preeching in a settlement is 3 and keeping agents actually in the settlement is important to level them up when not interacting with other Clans' agents . I'm not sure they can even convert without being in the settlement {I think not by memory} .

Place monks into a settlement whenever you're not using them to slowly rank up {it seems to be a progressive accumulation of points towards a levelup per turn , but I think it might have a random element to how many} and you should have enough to cover your needs if you build say one monistary per province {religion is important in this game} and use them wisely , build more if you want to use them more agressivly .