I don't know if the message means one or all of the princesses but it is clear that the prince and the princess think that it means only one. If they think that it means all of them then there are no point in the test that Elisa just took. Because they think that marriage with one princess is enough that Elisa try to marry Alec and check if the curse is broken.
I hope its all
Being only one would remove any sprinkle of originality It had (which is Sad to see for this Author which did an outstanding manga prior that got axed sadly)
And also would conflict with chiara Just saying shes ok with It ?
Because they could Just let she do it and everyone is happy and then divorce?
Or what everyone will sleep again If they divorce ? Lmao
Cant even argue the "oh shes gonna be a Princess with a poor reputation" because besides looking like she has a poor one to Begin with, everyone will sleep through the cerimony so they wont know lol?
So Thats my conclusion on why only one Just seems painfully stupid