count lookml with user input filter

hi,  i have created a measure with count and i have 2 questions:

1. on which granularity is the count performed? i havent set a primary key so i dont undestand how it performs the group by logic.
2. how can i set a filter that is given by user? for example filter: [in the last 5 days], i want the "5” and "days” to be input from the user