The minimum number of posts will depend on very specific requirements. In my area, and I assume many others, the local building department can provide very detailed guidelines. Decks are a big deal due to safety concerns. Your local jurisdiction may simply refer to other sources (e.g., International Residential Code) but they may have very specific requirements which you will need to follow in order to get your deck approved.
However, in many areas there is an alternative, a floating deck. This is different from a traditional deck in a few key ways:
- The posts sit on specially designed concrete blocks instead of being permanently attached to concrete footings. Actually, depending on the height of the deck, you may even have some of the frame resting directly on those blocks without posts at all. This eliminates the usual digging, concrete pouring, etc. and turns a deck into a truly DIY project.
- The deck is not attached to the house. That is the exact opposite of a traditional deck where not only is the deck attached to the house, but it has to be attached in very specific ways to ensure safety.
- The deck is height-limited compared to a traditional deck. But assuming "ground level" means "no part of the deck surface is more than a few feet above the ground", you're fine.
- No permits required, depending on the jurisdiction and the size of the deck. In my area, your deck is big enough that a permit would be required, but the permitting and inspection process should be simpler than with a traditional deck.
This is one example deck block from Home Depot:
The middle fits 4x4 posts, the slots fit 2x lumber.
Super-easy. Only tools you'll need are a circular saw to cut the lumber and drill/drivers to screw everything together.
In a quick search, recommendations seem to be on the order of a deck block (which may or may not have an actual post in it) every 4 to 6 feet. That is more than a traditional deck, but the posts here are easy and inexpensive compared to a traditional deck.