Looking on course WW to be slightly short of PS4 this year. PS4 already looking like it'll be fairly similar to last year, potentially ever-so-slightly less. (~19/20M).
With taking the above into account, and it'll be up somewhat YoY, I'll say 18M.
That said, there's potential for 20M+ (Pokemon, etc.), just the way it's looking right now, it'll be just below that, sold through.