The nm refers to the size of certain structures used to make transistors. Unfortunately each company uses a different structure as a reference, so you can't compare the nm number between companies.
- TSMC 12nm is about 29 million transistors / mm^2
- Intel 14 nm is about 35 MT / mm^2
- TSMC 10nm is about 52 MT / mm^2
- Intel 10nm is about equal to TSMC 7nm (both around 100-110 MT / mm^2).
- TSMC 5nm is about 173 MT/mm^2.
- Intel 7nm (if it ever gets here) is estimated to be at about 225-250 MT / mm^2,
- TSMC 3nm is supposed to be 70% higher than their 5nm, which would be about 290 MT/mm^2.
From what I gather, a silicon atom is about 0.132 nm across. So 1 mm^2 = 7.5 million x 7.5 million silicon atoms. Or about 325,000 atoms (surface area) per transistor. Or 570x570 atoms per transistor if they were square (less probably after accounting for dead space between transistors).
If you compare transistor density to process, the relationship is close to (but slightly less than) the square. So if you figure a 1 nm process results in about 25% the linear density of their 5 nm process, then you get transistors about 140 x 140 atoms across.