--为什么要动态的，传参数就行
declare @a varchar(20)
set @a='003,004'
select id,count(item) num from [Table] where charindex(','+item+',',','+@a+',')>0 group by id

------解决方案--------------------
SQL code
--楼主应该是查所有ID都有同一个ITEM的值
create table tb(id varchar(10),item varchar(10))
insert into tb values('A',                 '001') 
insert into tb values('A',                 '002') 
insert into tb values('A',                 '003') 
insert into tb values('A',                 '004') 
insert into tb values('B',                 '002')   
insert into tb values('B',                 '003') 
insert into tb values('B',                 '004') 
insert into tb values('C',                 '003')   
insert into tb values('C',                 '004') 
insert into tb values('C',                 '005') 
go

select id , count(*) num from tb where item in
(
  select distinct item from tb group by item having count(*) = (select count(*) from (select distinct id from tb) t)
)
group by id

drop table tb

/*
id         num         
---------- ----------- 
A          2
B          2
C          2

（所影响的行数为 3 行）
*/

------解决方案--------------------
--假设你要的数据在一个表中，表为A

SQL code
declare @sql varchar(8000)
set @sql = 'select id,count(item) num from [Table] where item in(select col from a) group by id'
exec(@sql)