If you want find How many Active Users in a Permission Set using SOQL query
use the Below simple query it fetches active users in Permission Set.
SELECT Count(AssigneeId) FROM PermissionSetAssignment WHERE Assignee.isActive = true AND PermissionSet.Name='<Permission set API Name>'
it gives the result in the form of Count.
Ex: if the count is Four, means 4 active users are in Permission Set.
Please Check Screen Shot for more details.
Please let us Know If you have any Queries.