Cannon Fodder Achievement
Recruit 500 crew members.
Your crew is important as they help you capture ships when you initiate a boarding action. You can upgrade the sleeping quarters on the Jackdaw to have more room for crew at any given time. You will have several missions where you recruit crew members, but most of your crew will be rescued sailors out in the ocean and recruited through taverns. If you manage to complete the game without recruiting 500 members - unlikely, but theoretically possible - you can attack and board a man o' war and let them whittle down your crew, then finish the capture and go recruit more at a nearby tavern.