Assuming you're taking about a disc harrow, it depends on the kind of disk. If it's a regular tandem disc with approx 18-20" blades then it's like paulharvey said, about 6hp per foot, maybe get by with a little less. If it's a heavy built offset type with large diameter discs you will need more power.
More than you would think in clay soil. MY 83hp (69pto) deere couldn't pull a 16' disc with any sort of speed about 4.5mph was its max and I was running out of ponies. Even had to downshift on the uphill. Constantly had to bump the hydraulic lift to keep from burying the machine. Disc wasn't set up properly but i sure didn't forget it was back there!
Around 90 with about 12,000 # of well balance tractor
Balance the tractor 60 rear 40 front so with any type of disc in most any land you can traction
The older bigger models from the 70s and 80s were much better discing tractors.