What do the numbers in brackets mean after City/Village on the RRT player aid card?

The displayed number indicates the number of said building type you need to connect in order to buy the resource in question, when playing with the Advanced economy.

For example: In order to be able to transport Food to a City, you need to connect two cities to the food plant or Stockyard.

Another example: you won't get passenger traffic until you've connected 4 villages to your line if you only connect villages. With cities, a point-to-point connection will do.

Try searching for "Complex Economy", or check the Manual (pp. 39, 50, 76-79). Quoting it:

The aggregate of supply and demand from several villages is needed to make rail service worthwhile