Mega is a million prefix.
If I have 1 million Ohm, I can say I have 1 MegaOhm. Million is 6 places, like 1,000,000 has 6 places: 1(6)0(5)0(4)0(3)0(2)0(1)0.
Milli is one thousandth prefix (1/1000).
If I have one thousandth (1/1000) of an Ohm, I can say that I have 1 milliOhm. Milli has 3 places, like 0.001 has 3 places: 0.0(1)0(2)1(3).
So if we add above together and I have 1,000,000.001, there are 9 places from 1 on the left to the place past 1 on the right.