I am able to see a count of users beside question in codechef long contest but I am not sure if that many users have earned full points.
Codechef doesn’t provide that feature but you can see how many submissions got how many points beside the problem. As for your query, one way is that you can use the ranklist and clicking on any problem name there will sort the ranklist for that problem only. But again, this will not give you the total number but you can count if you have time
The number in the ranklist seems to be axactly what you’re looking for (no of users with full points), except the challenge problem, where it is the number of users with at least one AC submission. Hovering over this number also reveals the number of users with partial scores
Just hold your cursor (mouse pointer) on the counts as @ceilks said , you will see both counts ( no. of users who have achieved full points and those who have achieved partial points as well ).