Count Distinct and SUMIF to find unique text values

Good Morning!

I'm aware Count Distinct doesn't exist as a function in Excel but I'm looking for some help on an alternative to return the number of unique text values based on one criteria.

I've attached a shortened example but I basically need to count the number of consultants working on particular projects.
So I have a huge time sheet and I need to be able to say 'X' amount of Consultants worked on Project A, 'Y' on Project B etc. etc.

I've managed to do the Count Distinct part - ={SUM(1/COUNTIF(I4:I12,I4:I12))} but not sure how to build in the criteria to say only for 'Project A'

Any help would be hugely appreciated
Kindets regards,
Laura