I can recommend Redmine. I have been using it for over 2 years, with 25-50 simultaneous users and more than 50 projects.
I went through a lot of updates without any problems.
The database is properly normalized, so if you ever need to get any data, you can do it.
There are many plugins that can cover special needs, if any.
Edit: At the same time, I had to switch to Jira, but I would return to Redmine at any time if I could.
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