18...people keep extra tees in their golf bags though...but there are 18 ON the golf field..as you call it.
This depends completely on the decression of the golf club. They may have no age limit on them, or they may have it at 18. However there is also the more likely option that there will be a handicap limit, which you have to have attained to play off them.
One round is 18 .. so 18 + 18= 36. ; 36 holes