Returning Row Count in a Stored Procedure in Oracle
You are almost there, just put the result of the select into the out variable:

SELECT COUNT(*)
  into v_EX_ROWS
FROM EXAMPLE
WHERE EX_ID = v_EX_ID


You might be better of using a function though:

create or replace function SP_INFO_EX_S(v_EX_ID    IN NUMBER)
   return number
AS
  row_count number;
BEGIN
    SELECT COUNT(*) 
      into row_count;
    FROM EXAMPLE
    WHERE EX_ID = v_EX_ID;

    return row_count;
END SP_INFO_EX;
/


Then just do:

select sp_info_ex_s(42) from dual;


Depending on your query tool, this might be easier to deal with than an out parameter.