Not a problem. Tell the Sales rep how many rooms you need. They will give you a deadline, after which they will release the rooms that your guests have not reserved. That deadline might be 60 days, 30 days or even a week before your big event, depending on how many of those rooms they figure they can otherwise sell.

In addition to the blocking of rooms you'll get a discount on the rate for the rooms.

Don't accept the first offer you get for the cut-off date on releasing blocked rooms without some negotiation.

Don't forget to tell the sales rep that you want your meeting planner points or miles.