How to set max user connection per database in mysql?
You may set a cron every minute to trigger below command and pipe it to a logfile and rotate it every day.

select current_timestamp();

select count(*) sumOfUsers , user,db from information_schema.processlist group by user,db;


After a day you may grep for values more than 90 in sumOfUsers column to see which line and then you may get timestamp above it. Thus you may measure time range and do manual monitoring around that time to see how often it takes the connections almost 100. Also you may add Host  in query to see if any particular application host is doing this pile up of connections.