Many applications for admission and scholarship applications require at least one recommendation letter. Some require as many as 3 (or even more). Always check the specific application's requirements as to how many and who the letters should be from.
I believe a good rule of thumb (absent specific requirements) is to have on hand recommendation letters from 3 people. In addition to teachers/coaches/club sponsors/etc., you should consider getting letters from non-school people. Some of these might be your employer/supervisor, neighbor, someone from your church, etc. Relatives are not appropriate recommenders.
Many applications will state specifically who the letters should be from; others may simply say how many to submit.
ALWAYS do the following:
1) Ask the person(s) well in advance of when you need the letter.
2) Complete the form below and provide all required information to the person you are asking.
3) Provide any additional info that person may need.
4) At some point, make sure you write the person a short "thank you" note.