Using this link https://www.united.com/en/us/baggage...or/any-flights
I created a mock domestic economy cabin itinerary for a Star Gold member.
You'll get one complimentary bag up to 50 lbs meeting the size limits (Up to 62.0 linear inches (157cm) L + W + H per bag).
A second bag is $40 prepaid or $45 paid at the counter.