We had a big party for our twins first birthday! We had both sets of grandparents, godparents and thier children, aunts, uncles and cousins. In terms of friends, I had a few close ones from mother's group and some old family friends too. It ended up being around 50-55 people on the actual day.
It actually worked brilliantly! My mother and a couple of close friends came round the day before, and we did most of the cooking. We made everything that didn't have to be done on the actual day, like the sponge for the cake, and the filling for the sausage rolls. The actual party started at midday, so we had plenty of time in the morning to make other food, like cucumber sandwiches, etc. We made sure there was lots of food for the older kids, and we had a few party games (pass the parcel, musical chairs) as well.
Of course, the choice is up to you smile I personally enjoyed having a large party, especially as it was the twin's first ever birthday. Having so many people there was also good for the kids, as there's always someone there to keep an eye on them! It also worked out for me because some of the cousins were at the 15-18 mark, so they kept the kiddies in check!
Good luck with the planning, and all the best for your son's birthday!