How many posts for a shed?
All structures in the US State of Georgia are required to be constructed in accordance with the International Building Code series*...the requirements of საქართველო in the Caucuses were not immediately available online, unfortunately.

General requirements for foundations are stated in the applicable code. Site specific requirements depend upon multiple factors and require soils analysis as well as accounting for whatever wind, seismic, and snow loads the code anticipates for your local.

Be aware that exemption from a permit does not equate to exemption from the building code...nor as a practical matter does the issuance of a permit or passing inspections mean that a building actually meets code...and as always the code only specifies the worst legally acceptable building.