Humanoid Regrowth Chamber: When a friendly character whose name includes "Humanoid" is KO'd, you may roll a d6 that can't be rerolled. On a roll of 5 or 6, place a character with the same name and point value on click #2 in your starting area.

I Can't Stop Your Brute Strength, But My Mind is Far Superior: The Leader can use Outwit. Once per turn, if he uses it to counter a power or ability that isn't a defense power, he can immediately use it again to counter another power or ability that isn't a defense power.

With Human Regrowth Chamber, if more than one Humanoid is KO'd with the same attack (say a group of them is hit with Quake), do I roll once for each character whose name includes "Humanoid" or just once for all of them?
You roll once for each character.

And what if I am using multiple Leaders and a Humanoid is KO'd... how many times do I roll?
When a character whose name includes "Humanoid" is KO'd, that will activate this trait. If you have more than one character capable of using this trait, each trait will activate. So you would roll once for each character with this trait. And, yes, that means that one Humanoid that is KO'd could potentially turn into 2 or 3 or however many Leaders you happened to have.

If I use I Can't Stop Your Brute Strength, But My Mind is Far Superior (now called "ICSYBSBMMFS" because that is a mouthful) to counter a special power or combat ability that let's a character use a defense power, what happens? For example, The Leader uses ICSYBSBMMFS to counter the Indomitable ability, which lets a character use Willpower (a defense power), can I then counter another power or ability or not?
The Indomitable ability is not a defense power. That ability lets a character use a defense power, sure, but the ability itself is not a defense power. So in a case like this, ICSYBSBMMFS can indeed be used to counter a second power or ability.