Each Center of Trade has a value depending of the quantity of goods exchanged in that place.
You can check where goods are going to be exchanged on the details of a province/colony/trade post or by checking colors when you are looking at the 'trade map screen'.
You have a limited number of merchants per year as indicated on the top of your game screen.
Sending merchants to Center of Trade costs you money depending of which Center of Trade you are sending them to.
Globally, far = expensive.
Each merchant present in a Center of Trade gives you an income depending of the overall wealth of the Center.
Each Center of Trade can support 20 merchants.
So, the more merchant, the more your income.
But, also check for other countries merchants. In Centers of Trade where competition is fierce (Genoa, Venice, Flandres typically), even if wealthy, you'll have a lower income because other merchants are also competing.
If you can set a monopoly in a Center of Trade (you need a specific level of trade technology I forgot), things are going better.
Until you can set monopoly, you can only have 5 merchants at best present in each Center of Trade.
So, your income is limited to 5/20th x your trade level x wealth of the CoT at best.
When you can set a monopoly, your income is :
20 - the number of foreign merchants x your trade level x wealth of the CoT
When you can achieve monopolies, you can also send more merchants to wipe foreign merchants from the CoT, so ensuring you a better profit.
Hope that does help.
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Pierre
Retired EU Beta Tester