I had no idea what it was based on, and I didn't even bother with the early estimates, as they were all over the place and there was little information on the virus to base any estimate on. I just see patterns in data, and the relationship between the population differences and the 2M number immediately popped out.
This is all incredibly speculative, as we have no idea how many people have been exposed. It is probable we will never know, given that fact that more than a few people do not develop antibodies specific to the virus. Obviously the number exposed is much higher than the number that reported cases, as many people are asymptomatic and many others will not get tested. Some experts are putting that number exposed relative to the number of known cases as high as 8x. If that were true, approximately 50% of the country has already been exposed, thus putting us in somewhere within reach of herd immunity.