The set will feature over 140 cards, making it the same size as XY.
There will be 11 Pokemon-GX, including Solgaleo-GX, Lunala-GX, and Lurantis-GX. Interestingly, Pokemon-EX have not been mentioned, nor have Mega Evolutions. Every set description for every set containing Pokemon-EX and Mega Evolutions has mentioned them in the past, even when Pokemon-EX were continuing from Gen 5 into Gen 6 with XY. So if they are in the set, it is odd TPCi is not mentioning them.
By comparison, XY featured six Pokemon-EX and two Mega Evolution Pokemon-EX. This means there’s three more Pokemon-GX in this set than there were Pokemon-EX in XY. To include Pokemon-EX, Mega Evolutions, and then 11 Pokemon-GX in this set would be very overwhelming! So based on these observations, it seems Pokemon-GX are replacing Pokemon-EX and that Mega Evolutions are gone (it’s also unlikely Mega Evolutions will be handled in the TCG in a different manner since no other new mechanics have been mentioned, unless they’re somehow incorporated into the Pokemon-GX mechanic). The trailers for Sun and Moon have been sketchy with if there will be new Mega Evolutions in the games, so the TCG may be further hinting there are not, especially since their dedicated mechanic is probably gone.
Sun & Moon will feature over 40 new Alola Pokemon, including some Alolan Pokemon like Alolan Meowth. This is the same number of new Pokemon as XY. The first XY set featured around 40 new Kalos Pokemon, Black & White featured over 70 new Unova Pokemon (when they were doing the whole “older Pokemon don’t exist” thing and had 156 new Pokemon to show off), and Diamond & Pearl featured almost 60 new Sinnoh Pokemon. This may be a large hint that Alola will have around the same number of new Pokemon as Kalos, since, generally, the TCG takes between three and five sets to introduce a new generation’s Pokemon. In other words, if the introductory number of new Pokemon is low, it could mean they don’t have as many new Pokemon to show off and don’t want to burn through them right away.
Sun & Moon will also see the release of two Elite Trainer Boxes featuring Solgaleo or Lunala. Like usual, each box will come with eight Sun & Moon booster packs, a pack of 65 card sleeves featuring Solgaleo or Lunala, 45 Energy cards, a player’s guide for Sun & Moon, six damage-counter dice, a coin-flip die, two acrylic markers, a collector’s box to hold everything, four dividers, and a PTCGO code card.
And one last bit of news! The set will hit store shelves on February 3rd, which is within a couple of days of when XY was released in 2014. This will be two months after Japan releases Collection Sun and Collection Moon on December 9th. At this point, it’s probably safe to say The Pokemon Company doesn’t want simultaneous set releases for whatever reason.
Update: Oh, I forgot about BREAK Pokemon. They seem to be gone too since they haven’t been mentioned in the set’s information.