According to Bloomberg, who have sources with knowledge of sales at Microsoft, the Surface has only sold 1.5 million units to date. Out of the 1.5 million, 400,000 of those sales are for the more powerful Surface Pro, while the remaining 1.1 million are naturally for the ARM-powered Surface RT.
While the overall number of sales isn't particularly amazing, especially comparing it to the 22.9 million iPads sold in the quarter ending in December 2012, it does show the remarkably quick adoption of the Surface Pro. The Pro was only made available in mid-February, meaning that all 400K sales have occurred in a one-month period, despite persistent stock shortage issues at the Microsoft Store and other retailers.
Meanwhile, the Surface RT was released in late October 2012, equating to roughly 225,000 sales per month since its launch; not to mention how the Surface RT is more widely available, including locations outside North America. It just goes to show that, unsurprisingly, consumers are more interested in a powerful Windows 8 tablet with full legacy application support, rather than a Windows RT machine with limited capabilities.