How many radio transmissions per hour occur on a given frequency over Europe and North Atlantic?
It helps to check prior research in that field, for example, VOCALISE: Assessing the impact of data link technology on the R/T channel (2005).


  Quantitative aspects of pilot-controller communication are addressed in two en-route air traffic control studies: the VOCALISE study in a real operational environment (Graglia and al., 2005), and the EEC study in a simulation environment (Hering, 2001).
  
  The VOCALISE study shows that on average 324 speech acts between controllers and pilots were recorded per hour in an en-route sector, or about 5 per minute.


(Source: Eurocontrol)

So for the en-route portion, a single sector/frequency deals with about 5 transmissions per minute. Note that some of the transmissions would fall under miscommunication, such as:


  
  Incorrect read-backs
  Call-sign confusions
  Initiation failure
  Understanding failure
  Memory failure
  Information failure
  Omissions
  Redundancies
  


An example breakdown:


  [Out] of the incorrect read-backs, 30% are simple "say again" requests, and another 30% requests for repetition or confirmation of route information.


So that's another aspect to include in the simulation you'll be conducting.