The following was copied word for word from the Europa Universalis forum and was done so because it might increase players understanding of the game.
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These are the factors for getting merchants that I know of.
(If you know something more please post)
Stability:
Stability;No. Of Merchants
+3 ; +3
+2 ; +3
+1 ; +2
+0 ; +1
-1 ; +0
-2 ; -1
-3 ; -2
If you have coastal provinces:
+1
If you have Protestant coastal provinces:
max +3
Attained trade level 5 and 10:
+1,+2
Domestice policy slider, Free Trade/Mercantalism:
+2/-2
Monopoly in Centre of Trade:
+1 per monopoly *
Centres of Trade owned:
+1 per COT owned
Notice!
You can never get more merchants than 12/year.
* (only as long as the monopoly is effective, the moment you loose the monopoly, you loose the extra merchant)
Its amazing what people do when they can't sleep...