Thank you for your informative response.
I never knew the uncertainty held about how many people were on commercial aircraft at a given time! I always thought there were people at the door with clickers?
The likelihood of the aircraft catching fire on starting engines must be deemed low enough that in the event that it were to happen then people who are managing the incident on the ground need not know how many people they should be expecting on the nose?
I appreciate that Ops would probably radio the info to the Rescue Services but I wouldn't feel right not knowing how many people were on my aircraft when the doors close! Guess you must just get used to it.