Some sink refers to some flow of data. (in some particular sound-server terminologysome particular sound-server terminology)
Without any sound server, an audio app would output its audio samples directly to the hardware audio device. No need of any sink concept in these cases.
The sink concept appeared with some particular sound server (never read it used regarding the jack audio connection kit sound server of choice.).
When a sound-server is used, the audio app sends its data to the sound server. This, according to its terminology makes one sink.
The sound server can output to the device, using that sound server terminology, this would make another sink
But it could also sink the audio samples to some post-processing application, itself sinking to the sound-server incidentally sinking to the sound device.
On a side note, that sound-server was absurdly made capable to also sink to… another sound server… :-)
From comment : "could the server output to more than 1 device the same (audio) data at the same time?"
Yes, with regards to the sound server having introduced the concept, it is possible thanks to its module-combine-sink, it would also be possible to sink the data output by the device, back as an input to some application or even to the sound-server itself thanks to the module-loopback (at the cost of a totally unacceptable latency though). Note that the same thing could be achieved more quickly without any sound server thanks to alsa virtual-devices..