As far as I know, the profit of all enterprises of the same kind will be reduced for 10% if you have more than 1 enterprise, and again 10% less for every other enterprise. I usually built no more than 3 enterprises of the same kind, this was the best cost/profit margin I could make. Maybe 4 enterprises and very rarely 5, if it was a really profitable business like dyeries in the Mongol regions, or iron works and tanneries in England/Northern Europe. However, the profit can still vary heavily. Therefore I used to close down businesses, if they provided very low or negative profit over several weeks in a row, and reopen the same business at some other place where I would get more profit. Once-high-now-low-profit businesses usually recover over a period of time, though.