I'd assume most of the big supermarkets have a 4-5 host cluster with the small local stores having a 2-3 host cluster. You've got the software to run the tills sure, but also the loyalty card system (which seems to have a local cache at each site based on how quickly it returns your first name), VoIP, Door Access systems, BMS, Digital Signage, Scan as you shop systems, CCTV, Stock management systems..

I can see how you hit 40k pretty quickly.