I assume that you mean that the maximum number of primaries in a day is 5 and max # of secondary is 3.
You could load up a table via excel for all the dates for the next 10 years. Set the Key field to be the date.
Then instead of having users chose the dates from a usual calendar date picker field, you would make a relationships where 1 date has many Primary Rep Bookings. Then make a summary of the # of Primary rep Bookings.
lastly, you would make a report for the form to use for the drop down list where the # of primary bookings is less than 5.
Then do the same for the secondary bookings. New relationship to the same dates table
