With 9 foot of depth and assuming you will only require 100' of 5/8" air line, the pressure required will be 4.16 psi.

Everyone is right about the cfm depending on the diffuser types. Rubber membrane be it disc or tubes require 0.5 cfm and the stones require 1.5-3 cfm.

A round, 1-acre pond should only required one diffuser station unless something like intense aquaculture is being done in the pond.

Best fishes! ><((((*>