I applied and was accepted in last year's cycle. I did not send in a letter of recommendation from every single activity I did. I think what they want are letters from the 2-3 activities which are most significant to you. If you were the leader of an organization, or spent substantial time doing research, then obviously you want a letter showing that you played a substantial role in these activities. If you volunteered in a clinic once a week, then that's not so important to get a letter for. Also, I did not send in a letter from the activity I was involved with last summer (it just felt too early to be asking for a letter).