I'm a lil confused and sorry if it was simple but I find it hard to understand.

lets assume I have n possibilities, which it starts from index 0 then why we say immediately at n-1 and not at n? to be more clear if I starts at index 2 then why I wouldn't say that I will end at n+2 otherwise we say would end at n+1? specifically I'm getting more and more confused once they saying to me you're now starting from index 0 and you have n/2 possibilities then we will end at index n/2 -1 , why not at n/2? exactly why starting from index zero will lead that we end at n-1 and not at n? thanks very very much