We have to be careful how we answer this, Tim R. Mortiss. There are, as Finagle pointed out, characters (largely unnamed) who are seen definitively to have been the victims of chestbursters prior to the scene in which they appear, plus, there are chestburstings that either occur but are not (completely) shown on screen, or are otherwise witnessed in ways such that the audience has to infer what has happened.
I propose the question, “how many times in the Alien movie franchise has a chestburster emerged from its host in full clear view of the audience?” The number, as you originally postulated, was pretty small until (as DWMarch noted) they upped the gore in Alien vs. Predator: Requiem. I count ten: [ul][li]Kane (Alien)[][unnamed Acheron colonist] (Aliens)[]Ripley (Alien 3)[]Rousseau (AvP)[]Scar the Predator (AvP)[]Scar the Predator (again, in AvP: Requiem)[]Buddy Benson (AvP: Requiem)[]Sam Benson (AvP: Requiem)[][pregnant woman in hospital] (AvP: Requiem)The Engineer (Prometheus)[/ul][/li]I don’t know that the other ones qualify.
In the cases of Ripley’s dream in Aliens, the dog in Alien 3, and the sacrificial maiden in AvP, the scene either ends before the alien emerges, or the crucial moment takes place off screen. Specifically, in Alien 3, I suspect the reason the alien emerging from the dog was not shown was to heighten the audience’s expectation that this would result in a different type of xenomorph (following the tradition established in Aliens of introducing a new creature with each film) to be revealed later.
The scenes in Alien: Resurrection probably shouldn’t count, either. Gediman surgically removes a chestburster from Ripley clone #8 toward the beginning of the film. They sew her back up and she survives to reach Earth at the end. When the alien bursts out of the last surviving sleeper on board the Betty, you don’t actually get to see it. The sleeper viciously pummels Dr. Wren, grapples him, and holds his body in such a way that the chestburster explodes outward through Wren’s skull, killing him. The survivors then saturate both Wren and the alien with gunfire, finishing them off.
The audience also doesn’t get to see the chestburster escape Sebastian’s lifeless body in AvP, either; Scar the Predator is watching it squirm around inside by using his infravision, but then the camera cuts to an external shot of the chestburster leaping through the air into Scar’s hand, where he then strangles it.
I bit the bullet (man, the things I do to fight ignorance!) and spun through Alien vs. Predator: Requiem tonight at high speed, watching the bodies pile up. The chestburster scene from the end of the first AvP movie replays at the beginning to establish how the Predalien is on the Predator ship. Then, the deer hunter, Buddy Benson, and his young son, Sam, are the first human victims - “impregnated” as Ripley calls it in Aliens - of the facehuggers that have escaped from the wrecked Predator vessel. The only other scene like this is the pregnant woman in the hospital who gives birth to a clutch of Predalien babies on-screen. The sheer number of aliens the military, townspeople, and Predator face imply that she couldn’t be the only host, though.
The scene in Spaceballs simply wouldn’t count because it’s not part of the Alien franchise.
Additionally, I should mention, we might have to come in here and command this thread to rise from its grave with the release of Alien: Covenant later this year.