PS4 20-24 million
Nintendo Switch 12-16 million (only because I think they are slightly more likely to hold onto a high retail price and their paid online service will be introduced which will put off many potential Switch customers. I would increase the number if a reasonable price cut is incoming)
Xbox One 10-14 million ( I feel Microsoft will likely price cut to maintain market share and the 'X' will get a huge price cut. )