It is sooo rich, that the serving sizes were pretty small. I would guess it would serve at least 15...but you could get more with even smaller servings.
I actually made the turtle trifle for Easter. I purchased one frozen pecan pie (2lbs...as indicated in the recipe). I let it thaw...and you are right: It didn't really stay in cubes, it kind of smushed together, but it still looked & tasted fine. I also subsituted frozen whipped topping...