Vacation calendar in Excel with restrictions

I am trying to make a vacation calendar in Excel that will prevent a user from entering more than the allowed number of people off per department per day.

I have a workbook that lists all employees and assigns them a department code letter, and the date on the calendar will auto-populate with the department code when they are marked off for that day. I need each day on the calendar to restrict the number of people off in each department. How can I achieve this?