The number of Servants is limited to 12 and 1 Foreman.
Once the general supply of Servants is empty, you won't be able to gain any more Servants. This can become tricky if you have a lot of Servants on the Great Wall, Decrees and your Ships.
Plan carefully and try to balance your Servant income with your Servant investments on the board.
In your first game, you may find that it's pretty tough to gain enough Servants. Your basic income every round won't be enough for all the stuff you'd like to do.
This makes the several "Servant engines" in the game important, i.e., converting 2 Travel Tokens into 1 Servant, converting Intrigue steps into Servants, Travel Tokens that give you extra Servants, Cards that gain you extra Servants, Decrees that gain you extra Servants, matching your cards with the Destiny dice.
Although it's perfectly possible to win the game without it, don't underestimate the power of the Foreman. Try it, and you'll see...