For that deep, clay soil, 3.
You could pull 5, but you won't be any deeper than a field cuiltivator, won't accomplish anything.
I was pulling a 10 shank, 2 inch chisel with 150hp, didn't always go deep enough.
Just got a 7 shank 4 inch twist chisel, and wow, that is cool, finally doing something in the dirt!
Bigger isn't always better. In good ground, I could have pulled a 12 of the old, or would be able to pull a 9 shank of the newer type.... But I don't chisel plow only to cover good ground, I do it to accomplish something, and that means more hp than needed some of the time, so it works in the tough ground.