From what I have been shown/told, Elsa will be crowned queen early on in the film.
In regards to the number of "actual" princesses, that always gets to be a fun little debate because it always depends on who is doing the counting.
Officially, it's Snow White, Cinderella, Aurora, Ariel, Belle, Jasmine, Pocahontas, Mulan, Tiana, Rapunzel and Merida.
But then the little issues arise in debates, such as then why "Princess Kida" is not recognized officially as one since she technically is. Guess when they started counting over there at the company they forgot her.
And then there is the debate over Mulan, how she fits in to the "princess" line up.
Ultimately, the "official" count will always come via the company on who they decide to toss into that "official" group, despite what any actual movie rolls are.