In most cases MCE grabs the channel list from the downloaded TV Setup Guide and location config (if that is entered wrong, then you could be missing out on channels), but it's also possible to do a scan search for channels too. I have no idea what the maximum number is though.
Since you mentioned Digital and Analogue channels, I remember in "Vista MCE" a lot of people ran into trouble with hybrid TV tuner cards, wouldn't surprise me if something like that was buggy in Win7 (there were registry fixes and tweaks to get it working on Vista), but even more likely than that, it's a "driver problem". You can get TV tuner cards working using drivers that aren't made specifically for your card and thus not "100% compatible" with your hardware...