The Foreign Trade Civic (one of the earliest of the Ancient Era) grants a Trading Capacity of one, meaning that your empire can have one Trade Route at a time. Trading Capacity is the maximum number of Trade Routes you can have at the same time. Traders may establish routes over land and sea, but embarkation requires Celestial Navigation.
In order to establish a Trade Route you need free Trading Capacity of at least 1 (that is, the number of currently active Trade Routes must be less than your total Trading Capacity), a Trader unit and at least 1 valid destination city.
