From Create Lists, you can create a report of patrons who have used the library within the last month. You will want to run this report as close to the beginning of the month as possible to retrieve an accurate count.
Find an empty Review File and click on the search Records button. The Query box will open. Set the Record Type to Patron, since those are the types of records you are requesting.
Leave Range set as it is (to search the broadest set of records possible) and start to build your query.
For the first line in your query, establish the patrons to those whose P Type represents your library, meaning your patrons.
You will only need one other line in your query, to determine that the patrons have used their library card within the last month. To do this use the CIRCACTIVE field
For Condition, use greater than, enter the date of the first day for the previous month. Leave Value B blank.
When the search finishes running, the number in the Current Records field will represent the number of patrons with a circ active date within the last month.