cj wrote:
greetings, i am building a deck that will be 17 feet wide by 9 foot
deep. i will use 4x4's dug three feet into the ground, with a bag a
cement in each hole. my question is how much support(4x4)will i need
for the 17 foot span? obviously one 4x4 on each end but will one 4x4
centered be enough or should i break the 17 into thirds? so 4 columns
instead of three?

Don't put the 4x4s in the ground. Consider sonotubes or equivalent with
rebar. Have the concrete foundation above grade by 3-6" and bolt the posts
to that.