Well first off thats a rediculous statement. For more fish you need another filter? Bogus ... what filter are you running on your ten gallon currently? I bet the one you have is fine. And regardless if you have a filter or not, frequent water changes will eliminate any issues of stocking capacity but be reasonable. You could ideally keep plently of fish in one tank with no filter but large frequently water changes .... its all in the science.
Andrew