create table a (id int identity,bcount int)
create table b (id int identity,aid int)
insert a select null union all select null union all select null
insert b select 2 union all select 1 union all select 1


select * from a 
/*
id    bcount
1    NULL
2    NULL
3    NULL
*/
select * from b
/*
id    aid
1    2
2    1
3    1
*/
update a set bcount=(select count(1) from b where a.id=b.aid group by b.aid)

/*
id    bcount
1    2
2    1
3    NULL
*/

drop table a
drop table b