I think the limit was per cabin, so like 4 or 5 items per cabin. I can't remember exactly. I did turn in 2 pillowcases and 1 photo mat for my cabin.
You drop off the items at Guest Services. They have a form you fill out, you drop off the item and something to sign with, like sharpies. Some also put candy or other treats in the bag for the CMs.
Hope that helps.