FMS and 15 licenses. Unless you decide to use IWP ( instant web publishing ) or CWP ( custom wen publishing ). I would suggest you read up on those and then decide if you want to go that route.
You may also want to consider using a FileMaker hosting provider, such as ourselves:
This would save you having to purchase the FileMaker Server license, as well as the costs of your server, maintenance, upgrades, etc. Also, note that we can provide substantial discounts on FileMaker Pro client licenses to our customers. Feel free to contact me here or off-list at jmay(at)pointinspace.com with any questions.