You are severely underappreciating how complex a retail organization of Tesco's size is.

I work in this space for a retailer almost the same size as Tesco and when factoring in all the attendant organizations, businesses, and functions it requires, 40k servers does not surprise me at all.

Outside of just the brick-and-mortar stores you have Marketing, Retail, eComm, Merchandising, Strategic Sourcing, FP&A, Finance & Accounting, Asset Protection, Corporate Real Estate, Retail Real Estate, Internal Audit, Supply Chain, Transportation, Business Services, Data Science, etc etc. and IT at every level of those. Each one of these components is large enough to be a medium-to-large sized company in its own right.