If you need to maintain a minimum staffing levels, you can use Timetastic's Maximum Absent function to set the number of people that can be off at the same time in a given department.
Setting the Maximum Absent is a function within Department settings.
Head to SETTINGS > DEPARTMENTS:
Select the number of users allowed off in the same period from the drop down against each department:
How it works
Setting Maximum Absent will limit the number of users who can book time off in the same period, within that department.
When booking, users will get a message if the limit is exceeded, that they can't make the request.
Include or exclude specific leave types
In SETTINGS > LEAVE TYPES (see: Setting Different Types of Leave) you can include or exclude specific leave types from the Maximum Absent checks:
Overriding maximum absent
Of course on occasions you may make special dispensation and allow someone time off which exceeds the threshold. This can be done by either the user's Boss or an Admin user booking in the time off on behalf of the employee.
When the Admin user or Department Boss logs in and makes the request on behalf of the relevant employee, they'll get a warning message, but unlike when they employee tried to book, they can override the warning.
Additional restrictions - locked dates
If Maximum Absent doesn't do exactly what you need, look at Locked Dates to set specific dates/ periods of time that users, departments or even the whole company can't take off.