Movies News Talk
Alien: Romulus is the ninth film in the series, but given its chronology, not every other movie has to be seen prior.
Although you should not see every single Alien film, it is strongly advised to view the original 1979 Alien and its 1986 sequel, Aliens. Romulus is an interquel set between these two Movies, hence it will probably draw on ideas, characters, and story points from both. Seeing these two films will help you to grasp the universe and the general narrative. Apart from that, Alien is a timeless movie that is quite worth seeing on its own.
You can skip Alien 3 and Alien: Resurrection without losing essential knowledge to grasp Romulus. Alien: Resurrection is a legacy sequel set more than 200 years later, while Alien 3 happens just after Aliens. Romulus's narrative does not revolve around these films, hence skipping them will not cause you to miss any crucial narrative device.
If you wish to see Ridley Scott's vision for the franchise and explore the larger universe, Prometheus and Alien: Covenant is something to see. Both of these are popular films with fascinating analysis of the Xenomorphs' and the Weyland-Yutani Corporation's beginnings. You can enjoy the film without seeing them, though; they have nothing direct bearing on Romulus's actions.
You could most certainly skip the Alien vs. Predator films. Considered the worst of the franchise, they have no bearing on the primary Alien chronology. Romulus is unlikely to mention these films in any kind, hence you can enjoy the film without seeing it.
Romulus is meant to be a stand-alone movie, hence even if Romulus is part of the Alien series, you can enjoy it without having seen any previous Alien Movies. But seeing the original Alien and Aliens will improve your viewing experience tremendously and help you to grasp the setting and characters.