Usually. Usually it's one gland that has the adenoma but you can also have a condition called parathyroid hyperplasia where all 4 glands are enlarged. You only need 1/2 of a parathyroid gland to function normally, that is, 1/2 out of 4 glands.
Typically it one. Here's a reference for you: 1: Ruda JM, Hollenbeak CS, Stack BC Jr. A systematic review of the diagnosis and treatment of primary hyperparathyroidism from 1995 to 2003. Otolaryngol Head Neck Surg. 2005 Mar;132(3):359-72. Review. PubMed PMID: 15746845.
Depends. Hyperparathyroidism is 85-90% from one gland but 15-20% of patients have more than one gland. That's why experienced surgeon is important.
Double adenoma. Double adenomas can be seen in about 10% of patients. It is important that your surgeon either uses intra operative PTH monitoring or explores all 4 glands to make sure there isn't a double adenoma.