How much Concrete in holes?

[Curious Homeowner]

I am building a deck and was wondering: This is probally a dumb question here but is there a certain amount of concrete that has to be in a 12"W X 30D" hole. I am digging 9 holes for the 12' X 19' deck. Can I put just 1 bag of mix then tamp the rest with soil? Does it matter?

[Concretemasonry]

Are you running the posts (what size?) into the concrete or are you using steel clips embedded into the top of the concrete?

Dick