How many devices run java?

The answer to your question - how many devices run java is -

There is actually some more detail on this on the Java website:

1.1 billion desktops run Java

930 million Java Runtime Environment downloads each year

3 billion mobile phones run Java

31 times more Java phones ship every year than Apple and Android combined

100% of all Blu-ray players run Java

1.4 billion Java Cards are manufactured each year

Java powers set-top boxes, printers, Webcams, games, car navigation systems, lottery terminals, medical devices, parking payment stations, and more.

Which comes from the Learn about Java Technology webpage. These numbers can be drilled down into a bit more (e.g. blu-ray sales, PCs with Windows installed 1, etc.) but the figure they give in the setup actually seems to be fairly conservative.

With regards to some specific claims, Blu-ray players have to support BD-J which is used for bonus content on Blu-ray disks which is why Blu-ray players must support Java.

One of the biggest areas of use of Java is in Java Cards which are smart cards containing embedded Java based programs that can be run on various devices. While Oracle cited a number of 1 billion units manufactured in 2007 back in 2008, others have noted total cumulative sales of about 5 billion units in 2007. This is another point that leads to the 3 billion devices claimed, perhaps being a conservative estimate.

In truth the number of Windows PCs sold by year isn't the best number, but generally OEM machines generally ship with Java installed by default so this is a soft number to work with.