The 18.5 million includes 4.1 million consoles that were sold during what Sony calls the “holiday period” which runs from November 23 through to January 4 (plus or minus a day depending on the region).
The last time Sony announced shipment amounts they reported 13.5 million PlayStation 4 consoles sold through to the end of September. This would mean that PlayStation 4 sales have garnered a 2 million increase since the July to September quarter (according to quarterly reports). Things must be going pretty well for Sony at the moment, especially since the PS4 already had a slight lead over the Xbox One.
Sony also noted that PlayStation Plus subscriptions surpassed 10.9m as of 2nd January. In terms of software, 81.8 million games have been sold (meaning roughly 4.4 games per system sold).
The PlayStation 4 went on sale back in November 2013 and is currently the fastest-selling console ever in the UK, outpacing the sales of the ever-popular PlayStation 2 (which ended up selling a monumental 150 million units worldwide).
Microsoft hasn’t offered an updated sales figure for the Xbox One yet; the last we heard was back in November 2014, when Xbox corporate vice president of marketing, strategy and business, Yusuf Mehdi, said that 10 million units had been sold to retailers.