1. You will need to purchase filemaker server or have a hosting company to host your database for 10+ simultaneous users. You will need Filemaker Pro or Filemaker Pro Advance to created or modify the database.
2. You would have the database located on a computer with filemaker server and then each iPad that connects could modify the database. You will need Filemaker for each computer that access the database on the server. The file isn't located on the iPad but on the server. You can put a copy of the database on the iPad, but it would be a standalone database that is not shareable.
I would start will Filemaker Pro Advance and design the databases before purchasing Filemaker server. It's a lot of money to spend, so you would want your database up and running first. You can have 5 simultaneous user with a databases hosted by Filemaker Pro or Advanced. Test with 5 users and then expand. Gives you time to learn filemaker and to see if it is going to work for you. Filemaker currently has buy one get one free until December 10, 2014 http://info2.filemaker.com/BOGO2014_offer.html
Thank you! I'll start with what you have suggested. :)