You ask for a letter from each teacher.
A) you invite the teachers to submit recommendations online via the Common App. When the teachers have done their recommendations, you select which colleges or universities you send them to, or
B) you give each teacher copy of the teacher recommendation form from the Common App (with your portion at the top already completed), and a stamped envelope, addressed to the admissions office at each college or university that you want that teacher's letter to go to. (For the return address, you put your teacher's name, and the address of your school.) The teacher will make an appropriate number of copies of his or her recommendation letter, and sent them to the colleges.