Is there such thing as multichannel digital tunner?
The answer depends on how you define "channel".
If you mean a RF channel (aka the major channel number, or the digits to the left of the decimal point in "xx.yy"), then the answer is "no". A tuner can only lock on to a single RF channel.
If you mean a digital sub-channel (aka the minor channel number, or the digits to the right of the decimal point in "xx.yy"), then the answer is "sort of, but really no". A digital tuner can only lock on to a single RF channel, but outputs all subchannels. It's a typically a software function external to the tuner to demultiplex this MPEG Transport Stream, and extract one subchannel for viewing and/or recording. Windows MC and most tuner applications and digital TVs can only extract & utilize one subchannel per tuner at a time. But MythTV can do a picture-in-picture of another subchannel of the same major channel as the main picture. There are some other programs that can record the entire Transport Stream (of one major channel from one tuner) for later demux and viewing/editing.
In order to receive multiple "channels", you should expect to install multiple tuners, either on multiple cards or multiple tuners on a single card (such as the quad-tuner in another answer).
If USB bandwidth is a concern (and it may well be with several tuners), then an alternative is an Ethernet network-based tuner, such as those made by SiliconDust. Their largest unit has 8 tuners.