How to find out AHU tr capacity when our AHU is running on 5000 cfm?

As a general practice 400 CFM/Ton can be used for typical office applications. 400 cfm/ton thumb rule is based on an average sensible heat ratio of 0.72 which is a nominal value.

It can also calculated from delta enthalpy (Mixed air- supply air) using a psychometric chart.

Total Load ( Q ) = 4.5 * CFM * Delta enthalpy