Yes, I'm reasonably confident that my friend knows roughly how many of her colleagues will be on duty at any given point. She can count, and she's fairly experienced, to the extent that she's been present at more than one "all hands" incident.
Are you seriously suggesting that Police forces are somehow uniquely incapable of managing or accounting for annual leave or sickness absence? Presumably not, in which case it's perfectly feasible to give an estimation of how many officers will be on duty at any given time. And no, I don't expect my friend will be able to give you a number off the cuff for the entire city, but she will be familiar with her own stomping ground, which just happens to include my address, and given that she knows more about how the force divides it's resources than I do, I expect she would be able to give you a ballpark figure for the entirety if you asked.
Regarding response times - all you need to do to get a handle on this is actually call for a non-emergency reason, which unfortunately I have had to do in the past for a number of different reasons. Sometimes they pitch up within the hour, but more frequently it's several hours, sometimes 12 or more. So what would you have me do? Go downstairs and ask a loud, aggressive woman to kindly wait while I request the police attend, and just hope PS are not particularly busy at that precise moment in time?