Assuming we are counting only national rail TOCs and that we use the normal rail planner definitions of what is and isn't the same station, I conclude that 7 is indeed the max and it's a tie between Doncaster, Liverpool Lime Street and Crewe.
I reach this from looking at http://projectmapping.co.uk/Resources/TOCs%20v52%20December%202019.pdf, looking for stations with a large number of lines and then counting just how many. While this map doesn't cover every station I believe it covers all the ones that are likely to be contenders for this title. Since some of the colours of the map are tricky to distinguish I cross-checked some junction stations on Wikipedia.
- Doncaster: 7
- Liverpool Lime Street: 7
- Crewe: 7
- Edinburgh: 6
- Glasgow: 6
- York: 6
- Peterborough: 6
- Manchester Piccadilly/Stockport: 6
- Leeds: 5
- Stoke: 5
- London Liverpool Street: 5
- Watford Junction: 5
- Wolverhampton/Birmingham: 4 (5 brands)
- King's Cross: 4 (5 brands)
- Sheffield: 4
- Southampton: 4
What happens if we also consider tram, metro and international operators?
Merseyrail is already counted because they are a national rail TOC, Doncaster, Crewe, Peterborough and york do not have any current tram or Metro system.
The Manchester Metrolink bumps Piccadilly up to 7 tying it with the lead. In Edinburgh, Haymarket seems to have a tram stop just outside the station but the sleeper doesn't stop there and the main Edinburgh stations is some distance from the tram stop. Glasgow has a subway but it does not directly connect to Glasgow Central.
But the big question is London King's Cross / St. Pancras. You have two clearly separate National Rail stations (separate buildings, separate in timetables, separate platform numbers), but they are both joined to the same underground station and I'm pretty sure it's possible to walk through the "outside gateline" part of the underground station to transfer between the two National rail stations without walking out into the street. If the whole complex was considered one station then it would have 6 national rail TOCs (7 brands) plus Eurostar and the London Underground for a total of 8 operators under 9 brands.