1. Maximum possible (active) heroes - only 5 instead of 6:
When I use the spell "summon Champion" I receive a pop-up / information window telling me that I may only have up to 6 (active) heroes and does nothing / gets canceled. Yet I only have 5 heroes.
I had one hero through the Mentor perk and recruted the others subsequently. Afterwards I wanded to get a 6th hero through the summon champion spell - only to receive said information - even though I only had 5 heros!
At that time none of my heroes had died, I neither used the hero or champion summonning spell before, nor did I receive another offer for another hero to join my ranks.
So I guess, for me, 5 heroes is the maximum.
2. Only 4 heroes or champions are summonable:
When I once again use the spell "summon Champion" the game tells me that I may only have up to 6 (active) heroes and does nothing / gets canceled. Yet I now only have 4 heroes still active / alive.
I managed to get (as stated above) 5 normal heroes. Afterwards by kicking out 2 of the heroes (1 at a time) through the army menue I was able to summon 2 champions.
Now in one fight one of my (normal) heroes died (and my sqadron / army won the battle). I did not used any resurrection spell on him (nor did I even have any such spell).
In the hero / item menue only 4 heroes are now shown.
In the army menue the ded hero is of course no longer visible / added.
Now I had started another casting of summon champion a few turns ago and expected to be able to summon the champion without further ado after the casting was finished in a few turns.
Yet, even though I now only had 4 heroes (alive / active) the summonig failed with the Information that I may only have 6 heroes...
MY GUESS: The dead hero is still countet as active by the System.
UPDATE for Problem 2:
I CAN(!) indeed hire more heroes (if they offer to work for me because of my fame).
Yet I CAN NOT summon(!) any more heroes or champions.
Despite the message that I may not have more heroes (despite now only having 4) I was able to get a 5th hero again. A hero came to me (because of my fame) and offered to work for me. I was able to hire him.
After adding the new hero (who offered to work for me) I dismissed him and tried summoning a champion. It did not work.
Only after dismissing this new hero as well as an old one was I able to summon a new hero / Champion.
Dismissing all my heroes and champions and summoning new ones I once again was only able to summon 4 units until the above message (that I may not have more heroes) appeared!
So now this seems rather like a bug / wrong setting in the spell System
Game version: 1.30
Windows 7
Prozessor: i5 -2500K (Takt: 3,3Ghz)
Grafikkarte: Geforce 980Ti