I have a, I'm going to say around 5 year old dog that is half collie half jack russle. I sometimes have around 15-20 people over my house, this is not something new I have being doing it since we got him. He usually just gets hyper and wants to play with everyone, but eventually gets tired and goes to sleep. The past two times though he does not get tired and he is biting many of my friends, badly. The bites are small but deep and painful. One friend was bit in the face and might have to actually get plastic surgery to prevent the scarring. A couple months ago he also started biting himself. It is usually just for the first 2 minutes when he sits down. I dont know what to do.

Any time your dog's behavior changes the first step is to take him to the vet to test him for medical problems... if he has lyme disease, arthritis, cancer, diabetes or another medical condition he could be in pain or uncomfortable and that could cause him to bite. Once the vet has ruled out any medical problems, your next step is to seek th help of a professional trainer or behaviorist who specializes in aggression.

I agree, the first thing you need to do is have him thoroughly examined by a vet, including a complete thyroid panel, in addition to the regular blood panels. Hypothyroidism can cause increased aggression, along with many medical problems that cause pain or discomfort.

Another thing I would be concerned about, because you said he's biting himself, would be a neurological problem such as focal seizures. The excitement and stress of having so many people in the house could trigger them, and one of the common symptoms of focal seizures is "fly biting", an involuntary snapping, that could result in a bite if one's hand or face happened to be in the wrong place at the wrong time.