Mysql show no more than 5 concurrent connection
The solution was actually very easy:

The Bitnami LAMP stack I'm using is configured with PHP-FPM, so the configuration is not on the apache side but in php/etc/common.conf

pm=ondemand
pm.max_children=5
pm.start_servers=2
pm.min_spare_servers=1
pm.max_spare_servers=3


inreasing the pm.max_children=5 solved the issue.