There are two numbers in parentheses listed after each tag in a game listing. For example:
The first number is the number of other people who added the same tag to the game. (Why bother adding a tag someone else already added? It's basically a vote for the tag: it strengthens the tag's association with the game. This improves the search engine by telling it that the game is that much more relevant when someone searches for the tag.)
The second number is the number of other games with the same tag. You can see the full list by clicking on the tag.