CHARINDEX With INT Data Type (Multiple Value)

I have a column that has 50 records and they are INT. I am trying to place them into a single parameter using the CHARINDEX using the IF Statement. I got an error saying convertinh from varchar to int type data.

Basically, this is what I did:
@RoomID varchar(max)

Then I declare it:

DECLARE @RoomId varchar(4)

SET @RoomID = 0

The IF part that I need to express this CHARINDEX function.

Then in Where clause I filtered it as:
WHERE r.roomid = @RoomId

Or what other way to handle INT data type and make them available for user to choose multiple value?