Updates to custom Wordpress booking plugin

Description

We have a custom wordpress plugin for a hotel booking website. Unfortunately the original developer is no longer able to help us.
There are a couple of things we need updating on the plugin. I appreciate this may be difficult without seeing the code but I can provide access to chosen people.
The system is allowing double bookings which we need resolved.
We'd like to prevent users from making more than 6 bookings per calendar month. For example, today I could only make 6 bookings for April, 6 bookings for May, 6 bookings for June etc.