Microsoft bought 666K addresses. How many addresses does ARIN even have now, and how likely are they to assign such a large block to a single entity at this point?
ARIN has 5.7 /8 equivalents, or 95M addresses available. Their policy is (roughly) to satisfy any requests where the addresses will actually be used in the near future ("50% utilization rate within one year").
Zero, and thus zero. No more IPv4 addresses exist; anyone wanting a new block of them would have to get them on the secondary market like this.
Edit: I stand corrected; while the root IPv4 namespace has no more available addresses to distribute to the regional registries like ARIN, the regional registries themselves still have some addresses left.
http://www.potaroo.net/tools/ipv4/index.html https://www.arin.net/resources/request/ipv4_depletion.html https://www.arin.net/policy/nrpm.html#four