To be fair, OKW gets the limit only because they're the only faction with non-doctrinal heavy tank.
But why not limiting all heavies to only one? What about the IS2, ISU or the Pershing?
British Churchill doesn't really count, given what it is.
Personally I find it hard enough to get one of each vehicle at all; either you never get the fuel for both, or the game ends before you can use the second one called in.