I wonder... A country that is free trade... should it have many cot's or little cot's...
Lets assume you have 1 COT, and 1 monopoly in it. You gain 1,5 merchants YEARLY out of it. It gives additional 5 merchants in 3 years and 4 months. Counting base value of +2 merchants, +3 from stability, and +2 from free trade, plus if you got plutocratic, additional +1, you gain this way maximum about 8,5 merchants yearly, which gives 17 merchants for 2 years, which is about 3 COT's, assuming that there is still some chance that you will fail to compete, or lose a merchant here or there, or gain additional merchant from events. Additional COT, give you 0,5 merchant, which mean you gain 5 merchants in 10 years from it. If in whole world there will be about 15 COT's, or less, you will not gain much by making additional COT, as it will slow you down, needing to fill additional COT. But if there is for example like 30 COT's it might be worthwhile to make additional, in order to gain some extra merchants. OF course, this calculation assumes, that you can keep trade in all those COT's over the world.
EDIT: So, if you are free trader, and there are less than 15 COT's or something around that number, and you can destroy a COT, you should consider it. However, if there are many COT's, keeping more COT's than others, can give you certain advantage.