11 managed by "ex Rotherham managers" in the sense they took charge after Rotherham

Barnsley
Donny
Exeter
Huddersfield
Leyton Orient
Lincoln
Mansfield
Posh
Reading
Stevenage
Stockport

5 more if you mean ex Rotherham managers in the sense they have previously managed us and at some point in their career they managed another current L1 team

Bolton
Bristol Rovers
Burton
Crawley
Wigan

I think....!!

Possibly add Rotherham to the list too...? Depends on the interpretation.