No of channels constant?

Sorry if this is slightly off topic but I don't know of any other place where
people might know the answer.

Is the number of channels an MP3 file is encoded in always constant? Or can it
change from stereo to mono somewhere in the middle?

Or to phrase it differently:
Can I always assume that pcm->channels has the same value within the output
callback as long as I'm working on the same file?

Thanks,
Erik