Count is normal aggr function, where you can count ant field, like
Count(EmployeeId) it will give you number of employees.
where getpossiblecount() works with selection, when you select a field, you see possible values for that field,
let say you select Department in list box, and on selection of Department you can see how many employees are in selected department, so you can use getpossiblecount() for the counting purpose. it will give you all employee for that selected department.
I was struggling to understand the difference between these two functions as I was designing a calculation condition on one of my charts. I was trying to understand which is the best practice if there was in fact a difference between these two functions.
As always, your posts help bring clarity when I'm a bit confused on something Qlik related. Have a nice weekend!