AWOL:
Not for digital TV, necessarily - a single frequency (multiplex) can carry multiple channels
It depends on the TV system and the broadcasters. There are still channels. The digital signals occupy (mostly) the same space on the spectrum that the old channels used. Channels 1-12 are in the VHF band and 14-69 are UHF.
The sub-channels, streams or whatever they call it where you are, do fit within that original channel and only require one RF tuner to receive the whole set. For example, the old channel 6 which now might actually be transmitting from a repeater on the UHF 19 channel will probably be sending "6" as an SD stream, "601" as an HD stream plus others. Most of the systems I know can get about 5 streams into one channel, depending on the mix of quality and compression ratios chosen by the broadcaster.