@Neiluj In our experience with the Cloudron platform, a single instance is not suitable for such a large user base for a particular app. Nextcloud itself is a beast. So many different use cases. So many so-called apps. With third party apps with specific purposes. And potential TB of data to backup.
If your use case has different branch offices with up to, say, 50 users, Nextcloud on Cloudron with Nextcloud federation and one Cloudron instance per branch office... that seems doable.
If you need a central Nextcloud instance with all functions that only scale upwards, choose a dedicated Nextcloud instance with support from Nextcloud partners.
Otherwise: One of our customers has a use case where Nextcloud on Cloudron with up to 400 users is not a problem. But this is a special use case with users who do not use Nextcloud with all its functions.
So my answer is: It depends.