20g x 1"=20
adult size 2" /20=10, so 8-10 is not overstocking.
Thats if you follow the junk 1" rule
50% pwc weekly split twice a week
40g rated filter

Corydoras are not big and really always hide except at dusk and dawn, they create a small bioload on the system but id compare a dozen cories to maybe 2-4 goldfish.