The readily available guidance is way out of date:
Up to 4 pumps - 2 x extinguishers minimum rated 34B (yes only 34B!) with foam being at least 9 litres, powder 4.5kg
For up to each additional 2 - another 1
I would expect there is some more up to date stuff in the industry, however what I see does reflect the above with most filling stations having two pumps per row, each pump capable of being used from each side so equal to 4 pumps and each row thus having a fire point stand of 2 extinguishers and sand buckets, but instead of using 34B rating stick to either 6 kg Powder or 6 litre AFFF Spray.
I do remember that you could have your preference of all foam, all powder or a mix as long as they were compatible - unless you had LPG pumps in which case everything should be powder.
The HSE guidance looks like it was based on the 1970's and Protein Foam & BC Powder extinguishers, with some adjustment in the 80's for BS5423 (adding ratings and metrifiying capacities)
