There is an engineering calculator that can determine the performance index of a system and what the impact of X amount of calls per hour with what applications you have. Your business partner should be able to run that calculation.
In addition to traffic, many other factors affect system PI. For example, a large number of voicemail ports can significantly increase the system PI because streaming data to the hard disk is a CPU-intensive operation. Similarly, call monitors (features, not voice) used for ACD, HotDesk, and several external applications, along with SMDR logging, can add processor load.These are all taken into account automatically in the System Engineering Tool.
IIRC with around 100 phones you are looking at around 10000 calls per hour on a MXe system.
Now I did notice you have 2 systems is the AX just for you analog and all you IP phones are registered to the MXe