I think the rumour about party size originated from the fact that you can now have 12 people at once in a party room (the 'looking for party' tool people use to find players to join groups).
This used to have a max limit of 9, which meant that if a group had a full party but needed a replacement potential party candidates couldn't get into the room.
Now the max limit has been changed to 12 they can. The max number in the actual party itself is still 9 however.