Limiting number of same entries in a column

Hi all

Please see attached agent schedule.

I want to limit the number of agents choosing their lunch start as "12:00" to 5 agents, "12:30" to 6 agents, "13:00" to 6 agents and so on...

is there a way to achieve this where if the 7th agent chooses his/her luch hour say "13:00" o'clock they get a message asking them to choose another time?

Many thanks

Johny