declare @c table (id int, lastdate date)
insert into @c 
select 1, '20181001'
union all
select 3, '20180905'


declare @a table ( id int, activity date, idclient int)
insert into @a 
select 34, '20180605',3
union all
select 53, '20180405',3
union all
select 777, '20180825',3


select C.id, C.lastdate, ClientActivity.Qty 
from @c C
	cross apply
	(
	 select count(*) from @a a where a.idclient=c.id AND a.activity>=DATEADD(dd,-30, c.lastdate)
	)ClientActivity(Qty)