If the 12.5m figure is correct, it would probably still hold the record, wouldn't it? The Apple II, TRS-80 model III, or the IBM-brand original PC are the only contenders I can think of.
I think the Raspberry Pi must be sitting at over 20 million sold by now.
They're technically on their 4th model by now though.
Actually, more versions than that as there's A,B and + versions of each too.
I don't think comparing a modern tiny processing platform to old fairly large desktop boxes achieves anything. An iPhone 6 has more computing power than even the Pi and it's sold 222 million.
The Raspberry Pi, despite its physical size, is a full-blown general purpose personal computer. There is nothing about a Pi that is fundamentally different from a desktop PC or, indeed, a C64.