Doncaster would have 8 - all 7 operators that stop there (Northern, TPE, LNER, XC, Hull Trains, Grand Central and EMR) plus Lumo who run through non-stop. Very occasionally, during engineering works south of Doncaster, Lumo terminate their services there for an onward connection on Hull Trains; on such a day it would hold the title of being served by 8 operators.
Peterborough has 9 operators if you count Thameslink and Great Northern separately (even though they are
legally speaking both just GTR brands).
If it's about photographing the largest number of TOCs in one location, rather than a physical piece of track/infrastructure that's shared by them, a location
like this (under construction?) building - overlooking the North London Line (NLL) and HS1 where they cross over Belle Isle Junction on the ECML - would probably win. You could score 9 or 10 TOCs, depending on the way you count the GTR brands:
- LNER, Great Northern, Thameslink, Hull Trains, Grand Central, Lumo out of Kings Cross
- Southeastern and Eurostar services out of St Pancras (also Thameslink out of the Canal Tunnels here, but obviously they can't be double counted)
- London Overground and Caledonian Sleeper on the NLL (CS are scheduled to divert this way in the base timetable during certain periods each year, so I'd have thought this counts)