Seems a little overcomplicated, you only need one loop that terminates when you get to the end of the sentence, and during the loop you can count the occurrences of the letter and the number of letters in the sentence, if you need that.
The loop variable itself could either be a pointer to the current memory location being considered, or it could be an offset. In C this could be written as
// look at str[i], which is *(str+i)
// look at *str
Do you have an example of a loop in the course notes you've got so far? Can you adapt that loop to do what you want?
If you can't see how, then post what you've got (8085 code isn't one I've met) and we can suggest what you might change to get what you want.