Questions related to licensing is a bit difficult to answer because Microsoft propose a bunch of plans with packages and options.
You can compare each plan/option from the official Microsoft comparison table to find the right model for your business requirements.
To answer your question, “should I buy 200 licenses?”
The answer is yes and no. It depends mainly of the subscription you will choose and for which users you want to give access.
Note: you can later on assign different kind of plans to your users according to your subscriptions. Which let you a certain flexibility to feet as much as possible your business needs and optimize your costs.
The best advise I can provide you is contacting Microsoft sales for guiding you.
Or mandating a Microsoft Partner which can accompany you and also negotiate Microsoft contracts on your behalf.