I would be surprised if there was any rule of thumb or method that you could use with any certainty. As with most call centre metrics & processes, there is no industry standard. Your particular problem is dependent on the nature of the business and the set up of the centre (e.g. hours of operation). With the limited data that you have, I would suggest calculating the average hourly calls & AHT for each day and running that through an Erlang calculator. The resulting agent requirement can then be multiplied by the hours of operation and divided by the hours that a typical agent works to give you the base capacity requirement (before shrinkage). Obviously, this doesn’t account for peak hour but will give you a fairly accurate indication of capacity requirements – which should be sufficient to cover for peaks. Alternatively, you could try estimating a % increase above the hourly average….but the only way you are going to be able to solve this problem definitively is to start capturing the info at an interval level.