I think its peak will be around 17m in one year, probably the year it gets an official price cut.
Just think, the system is probably gonna push 13m this year with no real next gen killer apps on its side other then next gen AC Unity, so last gens userbase has had little reason to upgrade other then hype. Imagine what Happens next year when we start off 2015 Q1 with The Order 1886, Witcher 3, Arkham Knight, swiftly followed by Bloodborne!
The lack of a Japanese market and stronger competition from the XO in 2015 forward will stop it from pushing past 20m in my opnion.