That's not too many fish for a 10g, I had 8 gold fish and a loach in my 10g and they did just fine.
I normally changed the water when I noticed it starting to get cloudy or filthy. Especially when the sides of the enclosure started to form nasty gunk on it (fish crap? LOL). All of my fish did just fine, and I even had a gold fish that lasted 6 years that was originally supposed to be a feeder fish for these huge fish my mom used to have (Oscar fish).
-EDIT-
Scratch that first part, I can't compete with Joey LOL. He's knows a ton of fish, but I just typed what I know from personal experience.
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