Forgive me if I am explaining things too simply - on a dartboard you can get the scores from 1-20 with one dart to those single segments; 21 you can get with triple 7; 22 you can get with double 11 - so the first score you cannot get with just one dart is 23.
This could go on all night I'm afraid,but I think the majority of ABers would have getting the gist of the question. Thanks for your correspondence,by the way.