For the PS4, it largely depends on how much Sony will cut the price. My prediction is a large range because of the uncertainty: 15m-19m
Coming from the most recent Xbox family shipments, the XBO appears to be flat. Sales should slightly improve in April-June since Batman + The Witcher >> Evolve + Hardline. It's really hard to say how many XBO's MS shipped last year, but I believe it was around 8 million. My range will be between 8m-10m.