How to calculate the needed capacitance of a start capacitor for a 12V 10A DC motor?
DC motors don't use start capacitors. You want a reservoir to keep the supply from sagging.
Use dQ = C * dV, or C = dQ/dV.
If you need 10A for 2 seconds, dQ = 20 Coulombs.
If you can tolerate a 2V sag in the supply voltage, dV = 2V.
Then C = 10 Farads.
Double check your specs though : if the no load current is 6A and the rated current is 10A, you probably mean 90A for the start current. Which would mean 90 Farads in the above.
And remember you can't just switch on a supply to a 10 Farad capacitor...