I just wanted to join the discussion since I’m facing basically the same question and this was the only thread concerned with best practices 
Do you maybe have any size recommendations for teams vs. channels?
For example, I’m currently part of a mattermost team consisting of multiple thousands of users of a large organization. My actual team (read: the people I work with directly) are about 30 people, where these 30 people work on different projects.
Would I now create a private mattermost team for my team containing multiple project-related channels or rather simply create multiple private channels (e.g. a general one for the whole team (“A-Team”) and one private channel for each project (“A-Team: Project Bloons”, etc.)) within the huge organization team?
Or are there any restrictions to consider that could help me decide?
Thank you in advance for any advice!