It is really up to your stamina, desire and time management. There are 6 or more character meets each day, plus the Princess Gathering (4 or 5 separate Princesses), the Frozen Gathering (Anna and Elsa, together usually), the Character breakfast (another 5 or so), and often times random meetings throughout the day (though I don't know if those will do signatures or not). Likewise, there will be multiple meets with Mickey, Minnie, Donald, etc. in different outfits throughout the cruise (regular, formal wear, pirate, etc.) so you have to decide if you want only one signature per character or ones for the different outfits (like if you plan to match the signatures with pictures).
So I would say that you should decide how much time you want to spend greeting characters, and how many pages you would like and plan according to that. You won't be short for opportunities.
Have fun!