Hi Jamil
A quick way to get the current active users (those who have logged in over the last year) is going to the System information page. or Totara registration page.
If by active user you mean those with an active status (not suspended or deleted) you could create a report using the User report source that included the User status and username column aggregated with count unique, this would show a count of users in each status.
The NPS calculation requires the percentage of users who gave a positive rating less the percentage of users who gave a negative rating.
This could be simulated by having a quiz with a multi choice question with the ratings as choices and each rating given a negative or positive grade . The grade calculation for the quiz would add the total positive grades plus the total negative grades to get a net positive or negative number which is what the NPS calculation does.
regards