That is somewhat of a vague question. I assume it means for a clan to down those bosses on a difficulty and ultimately it depends on the teams clan members are running and how much damage they're doing, respectively. My clan could probably do brutal with a few people using keys, and maybe 7-8 total keys on nightmare. 

If that's not what you were asking, please specify :)

For VictorTES just 1. :DDD

Number of Keys needed = Clan Boss Health Points / Average Damage Per Key

Where;

Number of Keys needed = K

Clan Boss Health Points = HP

Average Damage Per Key = D

K = HP / D

HP is known.  Therefore you need to calculate (or estimate) D.

That is excatly what I was asking. The problem I have is that my clan didnt beat the brutal yet. So I cannot do the missions for arbiter. So Im trying to find out how many keys are needed to kill brutal and nightmare.

You need to change clans...

Yeesh, being gated on Arbiter progression is rough. Might consider leaving myself, if I were in your shoes. 

However as to your question: it does become a math equation like Angwil laid out. 

This  is  very  simple.    You  need  to  do  361.55  M  damage  to  brutal.  Devide  that  by  your  average  damage  and  that's  how  many  keys  you  need  to  kill  brutal.