I actually don't know how many channels Disney has on YouTube. Certainly more than I personally watch. And I don't really care when any one of the channels launched, or any of the normal details I might try to provide about things like this. It's too complicated for me. So, I've just assembled the few that I do watch anything on, on this one page. If I start watching more things on other Disney YouTube channels in the future, I'll add them here, too.

Disney
This is, as far as I can tell, the main channel. Certainly it's the one on which I've watched the most shows (though that's still not very many). They include:

Dream Big, Princess
Star Wars: Forces of Destiny (part of this series is also available on the Star Wars channel)
Storytime (also on facebook)

DisneyD23 (channel no longer available)
Shows I've watched on this channel include:
When Did It Start for You?

Walt Disney Animation Studios
Shows I've watched on this channel include:
At Home with Olaf (also on facebook)

DisneyMusicVevo
I don't watch any shows on this channel per se, but I have subscribed to it and occasionally watch music videos on it.

Disney Descendants (channel has now become "Disney Channel Music")
Shows I've watched on this channel include:
School of Secrets
Wicked World (part of this series is also available on the main channel)

Star Wars
Shows I've watched on this channel include:
Rebels Recon