The default number of connection to the database is 50.
The ConnectionPoolingMaxActive is only limiting the maximum active instances in the pool.
So if the pool is 50 then setting the maxactive to a higher number will not result in anything.
From the apache documentation:
maxActive controls the maximum number of objects that can be borrowed from the pool at one time. When non-positive, there is no limit to the number of objects that may be active at one time. When maxActive is exceeded, the pool is said to be exhausted.
If you need to increase the number of database connections you need to talk to mendix support for this, but be aware that adding additional connections will probably need additional cpu and RAM resources as well.