Pressure Reducing Valves (PRV) locations?

You cannot install the PRV in the main riser. That leaves you with PRV's at the start of each lateral or smaller ones at each section or individual emitter.

I don't see any mention of pumps and emergency generators.

Our fire brigade can only service 19 floors. that will leave us with at least 6 floors with a higher requirement.

The best would be to contact your local fire chief and have him recommend the best or prescribed option for you.

i think you don't need PRV for your system , that because

1)fire piping are designed to overcome the applied water pressure

2) for fire fighting system , almost no water flow all the time, then no need to control pressure for no flow pipes.

3) in case of actual fire, you need full pressure to allow the biggest coverage area for all sprinklers

4) other way to control sprinklers pressure without using PRV is to use different pipe diameters for each floor ( it may be considered in your building design )

5) As other member said, it's better to consult chief fire man before taking your decision

You need to place one PRV to serve your low zone (Basement to 12th floor). It can be placed before the low zone riser. PRV on each floor will be expensive..

Have you considered pressure regulating hydrant valves.

Given your piping system is designed to handle the pressure, one of the solution to overcome the high pressure in the lower zones would be to use pressure-regulated hydrant valves. These deliver a constant pressure downstream of the hydrant (typically 3.5 barg) irrespective of the higher pressure on upstream side. These are available commercially.

Good luck!

PRV on each floor control zone valve to control each floor.