You need FileMaker Server 16 where you can scale your solution based on number of users
Then you need inside your solution use the Trigger OnFirstWindowOpen a script that checks whether there license is paid or not. This is quite complex and need a lot of considerations.
Then for syncing you have many things to think about. There are tools that can help you with that like GoSync, 360 Works MirrorSync and others. Syncing is complexed and to make sure each user have 100% bullet proof transportation in syncing you need to do a lot of error handling in the syncing part.
If you need someone to talk to who have done this in the past, contact a local FileMaker Business Alliance partner FileMaker Consultants, Data Consultants, Database Consultants
Because you have fixed users you may be able to use "FileMaker For Teams" licensing. Double check the licensing with FileMaker first, because you users aren't actually a part of your organisation you may not be able to use FLT licenses. If not then there is SBA (software bundle agreement) licensing that you can buy if you team up with an FBA partner.
FileMaker for teams and SBA licensing gives you a FileMaker Pro license for each user you purchase so they can access via Webdirect or FileMaker Pro.
Buy the number of licenses you need, and upgrade when more users come on.
If you're selling subscriptions and planning on having more than one paying customer accessing your solution files on the same server, you'll need to first be approved per the "Solution Bundle Agreement" program and be an FBA member in order to qualify for the hosting exception for FileMaker Server 15 and later. You can bypass this by spinning up dedicated servers for each customer or using FMS14. Also, your offline versions may not need to meet this requirement either.
You'll need licensing for the maximum number of concurrent connections you expect to have on each server. So, if you think 50 users might access the solution at the same time, you'll need 50 licenses. This is all assuming your using SBA FLT licensing.
At DocuWrx, we sell our Kosmas solution as a subscription, so have quite a bit of experience with this. There are a number of technical hurdles to jump through to meet customer expectations, so take that into account when developing your business plan.
If the potential user owns their own copy of FileMaker Pro Desktop, my understanding is that a concurrent connection is not required. Therefore you may want to have a dual pricing scheme ... one for a user that requires a concurrent connection and one that does not.
Thank you for all the feed back I am sure I will have a follow up question but going to read into some of the suggestions first. the replies reinforce my understanding that this is far more complicated then I would have hoped.
One thing I forgot to add to the first post all the users will have read only access. Not sure if this changes anything or not.