B.E.
1 gift should be fine. We had a few guests arrive with siblings and they only brought one gift. I had no problem with that, since my son was getting too many gifts anyway, as far as I was concerned. We did have triplets show up who each brought a gift, but when we attended their birthday party we brought a gift for each one too.
Around here $15 - $25 seems to be the average range for a gift. It's hard to get anything decent for less than $15.