How long is a piece of string? What sort of construction? Are these guys employed or 'subbies?

As the others have said, this is down to individual site risk assessment. The law does not necessarily require you to have anyone trained to fight fires, only to have an escape plan that allows everyone to escape without harm (amongst a lot of other things).

With that said, you may wish to look at fire fighting training in order to reduce property damage. No sense in losing a whole site to a small fire that could have been knocked down with a quick squirt from an extinguisher. To do this you will need to have your staff trained to assess a fire and determine if they are able to tackle it. Many fire wardens courses will do this but not all, so check course content. How many you will need will depend on shifts and how many areas you have on site. One person per area per shift is a good rule of thumb, depending on your definition of area of course!

If it were up to me, which it isn't, I would have everyone trained, but that is neither practicle nor cost effective but you don't know who will be closest to the fire or the extinguisher when it starts.

I would conventrate on good fire prevention and housekeeping, that way it is less likely for a fire to start, but when it does you need a small number trained in fire fighting techniques and everyone else needs to know what elso to do .... sound the alarm call brigade out, call the roll etc.

There is a difference however where timber framed construction is on the go.

Timber framed construction can burn rapidly therefore a higher number of trained people need to be available and with more extinguishers.

Different technique for a very different type of hazard.