SQL task - result single row - how to assign value from result to variable

I have a sql task, which is returning a single row (just does a select count(*) )

I need to get that count value into a variable then use it on constraints that come after the sql task.

I think its the query im using that is giving me trouble

SELECT COUNT(*) FROM DB.MyTable where testID = '(?)'

...where testID is a nvarchar(30) type

The error Im getting is something like

"ResultSet" property not set correctly, parameters not set correctly, or connection not established correctly.