Contact the graduate schools one week before the deadline to verify that all of your transcripts have arrived. Graduate schools will usually request three. If you can obtain four very good letters, do it. Professionals who know you best. Professionals who will be able to say good things about you.
Professors are best.
Employers are good if the work is related to your field. Supervisors of volunteer work are good if the work is related to your field. At least two weeks before the first graduate school deadline. Not more than six weeks before the first graduate school deadline. Your major and minor.