@Mdarkuka Qualtrics provides a few different types of ranking questions. Some of which you can add validation rules, however depends on question type used. More options here: Rank Order Question (qualtrics.com)
Assuming you are using the drag to rank, if a respondents only moves 3 of the 8, all 8 will have a value. Qualtrics won’t be able to tell that the respondent didn’t leave the 4th to 8th option there as they were also happy with that order.
I’d recommend using select, rank and group - or split across 2 questions with piped logic, where respondents need to make the selection of which 3 out of the 8 matters the most and then rank only the 3 which have selected. This is easier for respondent to answer, and easier for you to see their selection and ranking. Another alternative, is utilise the radio button or text box rank options, with these you can allow respondents to skip items or force response for all.