For my son's second birthday party in December, we invited three of his friends, plus their parents. I chose the friends he seemed to get along with best. They say only one friend for each year of your child's life, but three was a good number for us. You will have to decide if 5-6 would be too overwhelming for him at once, and then too overwhelming for you. Remember too that you will have the 5-6 kids plus the accompanying adults, so you will likely provide food for all of them.
We kept the party to 2 hours. When the kids arrived, we gave them time to play, just like a regular playdate. Then we had pizza and juice boxes for the kids (they sat at a little picnic table in the living room). The adults had pizza and soda, and a few dads had beer (my husband is into home brewing, and all the moms were pregnant so they didn't drink!) We also had a few other snacks which were available the whole time (before and after the pizza)--snacks were things like chip & dip, sun chips, pretzels. After we ate pizza, I brought out cupcakes--there were iced ones for the adults--the kids got to decorate their own. They LOVED it!!! I just bought two cans of icing, one chocolate, one vanilla, plus two or three different kinds of sprinkles and decorations. That kept them occupied and happy (of course there was adult supervision, but it didn't require much).
After that, it was about time to go, so the kids played a little more until the parents were ready to leave. I kept decorations to a minumum--partly because it was the week before Christmas, and partly because I just wanted to keep things simple. Favors were little paper bags I bought at Party City (red, for elmo). I put a Sesame Street character sticker on the outside of each bag, and wrote each child's name in black marker. Inside there was a small packet of goldfish crackers, a small pack of crayons, a can of playdough, and a sheet of Sesame Street stickers. (I got the full-size Playdough cans on sale at Target). I didn't do games because I figured it would be too chaotic, and they were still a little young. I figured it would be enough fun for them to just have unstructured play, and it meant less hands on for the parents, who needed the break!
Hope this wasn't too much info. Have fun!!!