Help me figure out how many "worlds" this is.

I'm working on a story that dives into the whole multiverse thing. In my story however the number of universes is finite, exactly 1 googol (1 x 10^100).

here's the difficult part, I want there to be a name for each universe using a system. my current system is this: after the first 999 universes alphabet letters are added (example: f-874),

and after z-999 another set of letters is added meaning it'll be double letters (example: ms-999)

so my question is, how many letters should be in the format so that each universe has a name? (I know i'm probably explaining this wrong, it is very confusing to understand but if you understand what i'm saying and know the answer or how to get the answer then please let me know)