You could technically fit 10 SD channels together in the US, except I recall that the government ("FCC") specifically REQUIRED (at least during initial rollout) at least one to be in highdef, unless you applied for a special waiver.

These days the FCC is unlikely to care, but I suspect viewership would suffer if any of the major broadcast networks downgraded their "main channel" signal to SD.

This explains the typical arrangement, as well as listing some of the extreme cases:

https://en.wikipedia.org/wiki/Digital_subchannel#United_Stat...

https://en.wikipedia.org/wiki/Digital_subchannel#Tradeoffs