As my title goes, how do you teh the ten symbol on Akuma's back of his outfit?

You did say it right. It is 'ten'

Huh? So that is how you say it? Ten?

yes, it is pronounced "Ten". Also, this is just some random info, in Japanese, that symbol/pronunciation "Ten" means "Heavens" or "Sky". Don't know what that has to do with Akuma

heh, I never knew that.

yea. funny, i learned that tidbit of info from DragonballZ

I already know what it means. It means heaven or sky referring to gods and gods are immortal. So the symbol for him is immortal. :)