Is there a global setting that will restrict the input of holidays to max 6 per week?

There is no way to cap the number of days entered as holiday. This is all about the authorisation process being incorrect or not adhered to.

Employees requesting holiday should request the full 7 days to block out the days on the rota and record days off by ticking the ‘Unpaid’ box. This will then not come out of their holiday allowance.

The person authorising the holiday should then check to ensure that the correct number of days are going to be processed and if they haven’t been entered correctly, they should not be authorised.

If the manager is entering for the employee they should follow the same process and tick the days off for that week.

The customer should ensure all users are trained in the process and I have attached a link to the community that explains day how to enter a day off.

https://fc.force.com/customer/s/article/WFM-Creating-a-Holiday

Hope this helps

Debbie