Halo: The Master Chief Collection will eventually support over 6.6 million user maps
There will be over 6 million user-created maps supported in Halo: The Master Chief Collection.
Over 6.6 million user-generated maps have been successfully copied for Halo: The Master Chief Collection, according to 343 Industries.
The user-created maps were made for Halo 3, Halo: Reach, and Halo 4. The current plan is to have each accessible when Halo: Reach is added to the collection (thanks, TheSixthAxis).
According to an official blog post from July on Halo Waypoint, while the maps themselves will be brought over, videos and screenshots will not. The team was unable to find a "workable solution" in that regard; however, it is still look at ways to bring over screenshots to Halo Waypoint and make them viewable and downloadable.
The maps will be accessible for PC users as well. A release date for the Halo: MCC games on PC has yet to be announced, Halo: Reach will be the first to drop. This will be followed by Halo: Combat Evolved, Halo 2, Halo 3, Halo 3: ODST, and Halo 4. That's the current plan at least as of press time.