How many kills is overpowered for spells?

How many kills is overpowered for RHA and UHA?

Regular Hero Ability = RHA
Ultimate Hero Ability = UHA

What you mean by how many kills?

Lets say the map im working on is footman wars. A spell like Blizzard can get kill 10 footmans and is considered overpowered and its easily abused.

It also depends on other factors: like level of hero, strength of enemies, number of enemies

And whether or not the spell is meant to kill units or heroes or whatever
Also cooldown is one of the biggest factors

it depends heavily on the following factors:
If the map is a "Hero Arena" type map (Mostly hero based)
If the map is a "Footman Wars" type map (Mostly massing weak units based)
If the map is a "Melee" type map (8 - 15 medium units, 3 or 4 units can overpower a hero on his own)

If its a Hero Arena type map then spells with a huge area effect but not much damage are useless, direct damage spells, holding spells and teleportation spells are the most useful.

If its a FM War type map then spells with an area effect are very hard to balance. If an area effect spell can kill all the spawn units in its area and still have a little left over, its way too powerful UNLESS its a special ultimate or long cooldown.
3 - 5 Kills for an area effect spell
13 - 16 kills for an ultimate

If its a Melee type map then you need a bit of everything
Almost kill for a direct damage spell
about half HP for a regular area effect spell
Almost kill for an ultimate area effect.
Kill for an Ultimate direct damage

i said 7-9 kills for a regular ability