As an example, looking for a minimum:
For a power plant with an oil tank farm the requirement following NFPA 850 would be based on:
- The duration of the fixed equipment involved in a worst water consuming reasonable design scenario (i.e. NFPA 15 spray cooling on walls of the involved tanks for 1 hr, the duration of the fixed NFPA 11 foam chambers (20 or 30 min)plus foam solution hoses for the diked area for an indicated time (I don´t remember it now). Plus 2hrs of 500gpm for hoses or monitors.
Depending on the particular design scenario, this would lead to capacities around 500m3 to 1000m3 for relatively small installations.
For tank farms in general, depending on the stored liquids and the geometry of the tanks arrays. The protection could be based on big monitors fighting fires for 2hrs or more, that, would lead to huge water demands.
If you dare to look for a minimum with fixed equipment like foam chambers, NFPA 15 spray cooling as the example above, and assuming that it is not your responsibility to provide enough water for manual operations for a brigade trying to solve a big problem, you may be looking for a design minimum and go for a less water design need as in the example. It is a difficult decision.
But of course I would not recommend to use engineering numbers to look for a minimum, especially if you have crude storage, and low flash point liquids, or a particular risky tank farm condition.
If you ask for it to industrial fire fighters, they would ask for fire pumps with the option to get water from a lake or the sea.