Is it necessary to have 2 heroes per castle?

I remember a long time ago, back in Heroes 3, my friends and I would discuss the game conditions beforehand – no holding down the "S" key in battles, no leveling up Diplomacy, and no taking more than 2-3 heroes for the entire game. :o Later, I saw "experienced players" in a club playing, and to my horror, they had a huge number of heroes! It seemed so silly to me then – it's inconvenient to control them all, it costs a fortune to maintain them, and there's simply no need for so many; they just get in the way. Heroes 3 and 5 have a lot in common – if you play for fun, to enjoy yourself, and level up a hero, you can play with just one. But without a group of heroes, without chains of troops moving across the map, it's impossible to win against a skilled opponent who can make more moves in a single turn than you can in a week.