I hope you can help me,

I have just built a spreadsheet for work, basically a register, what I am looking to do is when team leaders are booking holidays for their operators and marking it on the register, if I allow a maximum of 10 people off at any one time in my business area and someone tries to book an 11th person I would like a box to pop up saying the “maximum holidays have been used” thus stopping them over booking holiday.

Is this possible, if yes can you help please?