select
   datepart(wk,Subscription_Date) as WeekNo,
   sum(case when Subscription_Type = 1 then 1 else 0 end) as TotalSubcriptions
from tbSubscriptions
group by datepart(wk,Subscription_Date)
order by 1

WeekNo  |Total Subscriptions
21      |12
22      |13
23      |8
24      |18

WeekNo  |CumulativeSubscriptions
21      |12
22      |25 (=12+13)
23      |33 (=25+8)
24      |51 (=33+18)