There really isn't much fluff in the book so they won't need to cut much out. It seems two movies could be reasonable.

First movie: Introduce Bilbo and Gandalf (the movies have to work as stand alones from LOTR). The whole unexpected party thing will take longer on film then it did in writing. Then you have the trolls, then Rivendell and the trip up the Misty Mountains. I assume the will want to stretch out the scenes with the goblins and Sméagol. Then the escape and the worgs. Then meeting the bear guy whose name I forget. That is a lot of stuff.

Second movie: getting captured first by the spiders then the elves is going to take some time. It will take some time to tell the whole elf story and while that is going on they will want to introduce the humans. To make it mesh with the LOTR better he will need to play the elves more sympathetically and as grander creatures then book did. Then they have to escape the elves and sneak into the lonely mountain. Killing Smaug and the war over the treasure afterwards will take up a huge amount of time. Then you have to bring him home and set up the lead into LOTR.

Sounds like two movies easy and of course they are doing it for the money. You don’t spend hundreds of millions of dollars just to make fan boys happy, you want to make hundreds of millions more.