WITH Sell (Good_ID,PaymentSum,PaymentDate)
AS
(
  SELECT 1,50.00,'2018-02-26 15:37:00'
    UNION ALL
  SELECT 1,250.00,'2018-05-09 18:00:00'
   UNION ALL
 SELECT 100,500.00,'2017-12-31 23:59:01'
)

declare @table table( dt datetime, x int)
insert @table
  values('20180102', 1), ('20180202', 1), ('20190102', 1)
;

select year = datepart(year, dt), month = datepart(month, dt), count(*)
  from @Table
  group by datepart(year, dt), datepart(month, dt);

select year = datepart(year, dt), month = datepart(month, dt), count(*)
  from @Table
  group by datepart(year, dt), datepart(month, dt)
  with rollup;