You can make a select query that shows companies and their count of products like this:
Count(Products.ProductId) AS CountOfProducts
INNER JOIN Products ON Suppliers.SupplierId = Products.SupplierId
GROUP BY Suppliers.CompanyName
ORDER BY Suppliers.CompanyName;
Change it to a "make table" query if you want to store this in a table, but I'd suggest you leave it as a select query, that way, you don't have an extra table hanging around that need to be updated when the data change.