Maximising utilisation usually means an increase in latency. You don't want your ambulances or fire fighters at high utilisation.
The rule of thumb is that you want utilization to be where there is an acceptable latency depending on some percentile of cases. For a firefighter, you'd probably look at p99 latency. For a hamburger joint, p50 on order time would be good enough.
That's why I didn't call for it.
I believe OP's point is that you do want the staff of 20, for the once-in-a-lifetime fire that requires 20 people. See also: the recent Texas power grid debacle, or the saturation of ERs due to COVID.
No, you don't. Resources are not infinite, and at some point the budget has to be taken from other services which are more useful than a once-in-a-lifetime event.
Or you do want the staff of 20, but then it must be a volunteer and/or on-call duty system. Otherwise it is not sustainable.
In my country, the shift/switch to professionalisation that started 30 years ago has gradually become a big problem. Despite the fact that they still represent only 20% of the firemen, they are killing the budgets, and they always want more (lots of strikes); apart from 'standard' raises, the most common thing they ask for, is that on-call hours should be paid full-rate, as active hours. Which, beside being extremely costly, is absurd when they are 'working' 24h shifts! It contains a few hours of training and, depending on location, a few hours of duty; the rest is on-call (at home or on premises depending on the type of station), the number of service calls is limited, and 1 in 4 shifts happen without a single call (even more for 12h shifts).
There are plenty of other problems which surround this professionalisation, but they are not directly related to this subject.
If you get paid the same, responding to a call just becomes an annoying interruption ("I could be at home making the same money if this person I'm helping had just been more careful!"), leading to a worse experience for everybody.
The general point, I think, could be summarized as: you want some idle resources to handle surge capacity. Whether that’s 5 or 20 firefighters is of course the next question.