Harvest Right Greenhouse
For the 24 foot greenhouse, we recommend 15 anchors (one for each hub). With rockier soil, have at least 900 lbs per anchor -- it wouldn't hurt to go up to 1,500 or even 2,000. The anchor manufacturer should be able to provide guidance as to your specific soil conditions and how to reach at least 900 lbs.
American Earth Anchors
One of Harvest Right's recommendations, I spoke with AEA and they recommend the Penetrators (PE-18) with tie-off -- or PE-26 with tie-off.
If the soil is quite solid, consider drilling a 1" 'pilot hole' before anchor to ensure anchor is in. Any impact wrench with a 1" socket should be able to drive these into the ground.