Usually we look at the noun being modified to determine whether there is one item or many: boat race or boat races. Even though one race has many boats, we use the singular boat when modifying the noun race. This tells us what kind of race, but not how many boats.
However, there are some idiomatic exceptions that use a plural noun as an adjective (news reporter, sales team).
Incorrect There were fifteen boats competing in the boats race.
Correct There were fifteen boats competing in the boat race.
Incorrect There is a lovely apples tree on the corner.
Correct There is a lovely apple tree on the corner.
Incorrect The crowd could have filled a sport arena.
Correct The crowd could have filled a sports arena.