@YurkshireLad The memory recommendation is the absolute minimum memory below which the app won't run reliably. Like even with 1 user, it won't run properly below that recommendation.
The amount of memory required to host 100 users... is hard to know. Usually, the answer to this depends on the how all these users user Rocket.Chat. If they use simultaneously and if they use it from multiple devices etc. Cloudron will give you a notification (email) if an app is running out of memory. So, you can just take it slow and increase the app's memory limit as you go. You can also look into the upstream project's memory recommendation for 100 users.