Nothing* outside of the class rate increases number of attacks.
*yes, yes, I know heroism exists. That doesn't do any counter arguments any favors as that simply increases the gap in the Ninja's favor.
I would like to correct the official documentation and clarify how
Heroism works in MM3-4-5.
It simply adds the spellcaster's level to the "To Hit", aka "Attack bonus".
The manual AND in-game description for all these game is wrong. It does NOT increase the level of the physical attacker.
Allow me to demonstrate with this test from MM3 with a level 1 barbarian.( I can't include my screenshot here sorry)
My mid-game level party dumped the knight and I created level 1 barbarian with 13 might, just for this test, and I equipped him with steel hand-axe (roughly 13 damage on average). I went to the Arena I checked the monsters' hit point before and after the barbarian's attack (using the Where are We mod). Only 1 hit. Every time.
So if the official description of the spell was correct, my cleric's Heroism spell should have given more attacks to my levl1 barbarian. But he attacks only once. EVERY time. For roughly 13 damage.
The Where are We mod folks know this, and corrected the spell description to:
"+Level to hit on one character(mm3)"
"+Level to hit on one party(mm4-5)"