The Hills Have Eyes

At a run-down gas station in the middle of the desert, an old man named Fred (John Steadman), is packing his truck when a ragged and somewhat feral teenage girl named Ruby (Janus Blythe) approaches him. She offers to trade what she has in her bag for food, but the old man refuses. They walked into a small cabin and Fred scolds her for what "they" have done. Ruby says that her family ambushed a nearby airfield because they were hungry and that no one any longer passes by where they live. She pleads with Fred to take her with him, but he refuses and warns Ruby that she could be in danger if "the pack", in particular someone named Jupiter learns what she's doing. She replies that his life will be in danger as well if Jupiter finds out that he's trying to leave. A noise distracts them and Fred orders Ruby to hide.
The Carter family is traveling on vacation towing a travel trailer from Ohio to Los Angeles. Parents Bob (Russ Grieve) and Ethel (Virginia Vincent) are driving, accompanied by their teenage children Bobby (Robert Houston) Brenda (Susan Lanier), eldest daughter Lynne (Dee Wallace), Lynne's husband Doug (Martin Speer), their baby daughter Katie, and their dogs, Beauty and Beast. They stopped at Fred's Oasis for fuel and Fred urges them to stay on the main road as they leave. Fred's truck suddenly explodes, preventing him from leaving. Ignoring Fred's warning, the Carters skid off a desert road and crash. Bob walks back to Fred's Oasis to get help. The dogs become very panicky and start barking at the hills. Beauty then runs off into the hills, where she's slaughtered by someone. Bobby chases after her and finds her mutilated body. Frightened, he runs and falls and knocks himself unconscious.
As night falls, Bob reaches the gas station, where he finds Fred, who has tried to hang himself. Fred tells him about his son's family of deranged cannibals who live in the hills through which the Carters have traveled. They are commanded by Papa Jupiter (James Whitworth) who had killed his mother, Fred's wife, during childbirth. As a child, he killed the livestock on his father's farm and later murdered his sister. Fred attacked his son with a tire iron and left him in the hills to die. However, Jupiter survived and had children with a depraved, alcoholic prostitute known as Mama (Cordy Clark). Together, they had three sons: Mars (Lance Gordon), Pluto, (Michael Berryman) and Mercury (Arthur King), and an abused daughter, Ruby. They survive by stealing and cannibalizing travelers. Papa Jupiter suddenly crashes his hands through a window and kills Fred with a tire iron and takes Bob prisoner.
Brenda finds Bobby, who is shaken up about Beauty and returns to the trailer but doesn't mention Beauty's death, not wanting to frighten the rest of the family. He then gets locked out of the trailer and asks Doug for his keys, but is unaware the trailer is locked because Pluto is looking through their valuables while Ethel and Brenda are asleep. Before Bobby enters the trailer, Pluto signals Papa Jupiter to set Bob on fire on a stake as an explosion is heard in the distance. Ethel, Lynne, Doug, and Bobby rush out to save Bob while Brenda stays in the trailer with the baby. As they extinguish the fire, Pluto and Mars ransack the camper and Mars promptly rapes Brenda. The Carters eventually extinguish the fire, but Bob dies shortly afterwards. When Ethel and Lynne return to the trailer, Lynne is attacked by Mars and Ethel hits him with a broom. After Mars shoots Ethel and Lynne, Pluto abducts the baby and the brothers flee. Hearing their screams, Doug and Bobby rush in the camper only to find Lynne dead and Ethel mortally wounded.
Mars and Pluto return to the cave, but Beast pushes Mercury off a hilltop as he falls to his death. Ruby is then chained outside the cave with Mama tormenting her and is forced to eat Beauty as punishment for her betrayal.
The next morning, shortly after Ethel dies, Doug sets out to find his baby while Papa Jupiter and Pluto set out to kill the remaining family members. Beast tears Pluto's throat out and Papa Jupiter is killed by a trap set by Brenda when Bobby uses their mother's corpse. Doug sees Ruby knock out Mama and escape with Katie into the hills.
Doug catches up with Ruby and the baby, but Mars follows them. As he tries to kill Doug, Ruby interferes by putting a rattlesnake on Mars' neck, enabling Doug to overpower him. Doug then repeatedly stabs Mars to death while Ruby weeps over her brother's body before the screen fades to red.

Question: How many children did Mama and Papa have?
Answer: 4