said by Guspaz:
Call them what you will; decoders, tuners, either way you can only handle as many streams as you have physical decoders. No STB does decoding in software (I'm sure there are exceptions).
You're thinking about this the wrong way.
It only needs to be able to "decode" one stream, as one can assume this is the maximum number of video sources that the box is able to output. If it has the ability to display different things on different outputs, you would need one decoder for each independent video source as it is sent to a different output, though I doubt that is likely considering the type of hardware we're talking about.
As for "streams" it would be wholly correct to say that you would be limited by the speed of your Internet connection and nothing else. The information being received by the box would be saved to the hard drive in native compressed format, and only decompressed and decoded upon viewing. As a result, decompressing and decoding what is being received by the box and saved to the hard drive is a non-issue as far as box hardware and instead has everything to do with your net connection (or the Ethernet controller in the box, but I digress...)
The whole idea of "tuners" or to think of decoders like tuners in this case is meaningless, because there's no physical channel RF to tune into like there is with a cable DCT and satellite boxes. Cable and satellite PVRs save data directly to the hard drive in the native compressed format, so in those cases you only need one decoder, but you also need as many tuners for each individual stream you're saving.