sum of count in where clause (or having) in the same table with group by
First limit the result to 500 (or whatever) and then GROUP BY:

SELECT 
    binno,
    count(*) AS total
FROM 
    ( SELECT binno
      FROM products
      WHERE location = 1 
        AND sku = '4'
      ORDER BY binno             -- the order may be different
      LIMIT 500
    ) AS lim
GROUP BY 
    binno ;


Optimal index for this query:

INDEX (location, sku, binno)