Although there are still hundreds of 747s including both -800 and -400 variants. what makes this rare includes many factors. There are around 40 Boeing 747-422 aircraft around the world, which is a passenger variant. But only one of those flies in the US. Atlas air has 4 designated for VIP, And 1 for passenger. This is very lucky and I’m glad I could see it. If any of my information you read sounds incorrect these are my sources (https://www.airportspotting.com/united-airlines-boeing-747s-where-are-they-now/)