All that you can do in these circumstances is get agreement on a set of assumptions required to determine a busy hour traffic level for sizing purposes.
One set would be:
1. Number of working days per month. Also important to know information such as ‘we expect 10% more traffic on Mondays’. 2. Proportion of daily traffic received during the busy hour. 20% is often used as a conservative guess.
Main thing is to get agreement to your assumptions – good luck!