Hi Anh,
Looks like a variation on this formula Michael shared. For the following, set up a widget or dashboard filter on Season Fiscal Current Year Offset = 0, limiting the constituents to current season buyers. Add another filter on something that limits both the current season buyers and our interrogation of their historical purchase, to subscriptions... for example Is Package Flag = Y. Alternatively you can try Price Type Category = Subscription.
SUM ( [Constituent ID] , IF ( ( ISNULL( ( [# unique Package Seat Key] , [Season Fiscal Current Year Offset < 0] )) OR ( [# unique Package Seat Key] , [Season Fiscal Current Year Offset < 0]) = 0 ) , 1 , NULL ))
For each constituent, explore their package seat count in any season fiscal year prior to the current one, and if there isn't any history prior to this season, then include them in the total count. It would probably work with just the INSULL and without the OR = 0, but this covers the cases.