We have got the problem that Lime Survey Web is not available with error message " Internal server error CDbConnection failed to open the DB connection"Then we checked the error log, we found that the server has reached the maximum size allowed in “MaxClient” setting”
And at that time the maximum transactions are 5900 transactions when we got this problem (As below image/picture)
How to set/adjust maximum client or how to fix problem ?

There error message of Limesurvey doesn't relate directly to the max client warning of the apache webserver.
The limits of the database and the OS should be checked too. Since the DB connection limit is not the same as the apache limit.
Your administrator should know how to configure Apache. This is beyond the scope of the Limesurvey forum.