We tried RocketChat for about a day before switching to Mattermost. It wasn't great, but worked okay for a team of 20-30.
After a month or so of mumbling and grumbling on MM we went to Zulip, and brought on another dozen or so designers and PMs. Zulip lasted about 6 months before management implemented Slack company-wide and rolled out for the other 100 sales and support teams.
Zulip was the best for engineering, by far, but it was doomed from the start because the UX wasn't as simplified and polished as Slack. It didn't Just Work, you had to think and use it properly to see the benefits.
Back to your question: no limits I know of, and a t2.medium was 90% idle for an active team up to ~50 without a hitch. I think the docs (or launch announcement?) mentioned a large deployment around 1k, but I can't remember the details. You'll definitely want to test your expected configs and hardware to be confident.
After a month or so of mumbling and grumbling on MM we went to Zulip, and brought on another dozen or so designers and PMs. Zulip lasted about 6 months before management implemented Slack company-wide and rolled out for the other 100 sales and support teams.
Zulip was the best for engineering, by far, but it was doomed from the start because the UX wasn't as simplified and polished as Slack. It didn't Just Work, you had to think and use it properly to see the benefits.
Back to your question: no limits I know of, and a t2.medium was 90% idle for an active team up to ~50 without a hitch. I think the docs (or launch announcement?) mentioned a large deployment around 1k, but I can't remember the details. You'll definitely want to test your expected configs and hardware to be confident.