To achieve a one million - to - one ratio, requires 20 bits.
No. Imagine you have a candle and an airplane's landing lights. You only need 1 bit.
0 == 1 lumen
1 == 1,000,000,000,000 lumens.
That's a trillion to one contrast ratio with one bit. If you want to be really pedantic contrast ratio isn't actually interesting because while 1:1,000,000,000,000 is a high contrast ratio, it's actually impressive because of the dynamic range not the ratio.
0 = 0 lumens
1 = 1 photon
That would be an infinite contrast ratio. And also expressed by one bit but not what most people think of when they think of high contrast ratios.