Hey @%{original_poster_username,fallback:there}, welcome to the Nextcloud community 
Your project sounds great!
While Nextcloud is open source and the sky is the limit as far as user numbers and scale are concerned, your use case is beyond the community’s scope. The community forum is a community of volunteer enthusiasts helping and supporting fellow users. The community has a huge knowledge base and some clued up folks who gladly share their knowledge and expertise in the spirit of open source. Community support is not affiliated and in no way obligatory!
Mission critical deployments require professional consultants, engineers and supporters. Boundaries and limits are fluent and there’s plenty room for interpretation. The recommendation suggests, from ~100 users you’re looking at a corporate instance and from ~500 users you’re looking at an enterprise solution. Your best option is to talk to the folks at Nextcloud or affiliate partners. They’re certainly helpful and inclusive and will not turn you away.
You’ll find community documentation and deployment recommendations here;
You’ll have access to concise expert documentation upon subscription here;