Count(Total Distinct WEEK) should give you the total number of weeks after selections.
If you have a chart with Sales Reps, then Count(Distinct WEEK) should give you the number of weeks each has in the data, then another measure could be Count(Total Distinct WEEK) - Count(Distinct WEEK) to give you the number of blanks weeks for each sales rep...
I may be completely off but I'd need more information or a data sample (with expected results) to help you better,
How do I write an expression so that qliksense knows how many weeks are available in the selection? ie if we are 8 weeks in, how does Qlik know its 8 and see a sales rep only had activity in 5 and so has 3 blank weeks?
Do you have calendar in your data model?
If you have generated Master calendar by considering Min Date and Max Date from your Data then, in master calendar, all weeks will get generated and according to your selection of Sales Rep it will give you number of weeks for that SalesRep.
So expression given by luismadriz should work.
Still It would be better if you provide sample data and expected output.