1,900 were hired in Seattle alone. I saw that recently on /r/Seattle. Considering their orders are reported to be 40% year over year, 15k - 3k IT staff = 12k x 1.4 ^ 5 is 65k. They only have half that staff total so their efficiency and scaling improvements in distribution means they have to higher less people year over year for warehouse staff.

This is all uninformed speculation. The only public number I've seen is 1,900 hired in Seattle over a 3 month period. That means a lot more than 2k were probably hired for development, which means that Amazon is definitely up to something.