DISCUSSIONS on “Rules for online games in HoMM3 and formulas for calculating player rating.”

Most of them – how many? Roughly – about 5 matches, no more. Tournament games with elimination rounds.
In seasonal leagues and tournaments with a round-robin format, a win isn't necessary.
Is 5% really a majority?

"Majority" means almost all tournament games. In single and double elimination – absolutely all, and in round-robin and Swiss-system tournaments – a significant portion. Simply because usually only one place matters – the first, meaning a draw is generally equivalent to a loss and only gives an advantage in sharing additional places.

I'm not considering seasonal leagues, as the result isn't the main thing there.