In a two-mirror interferometer, only one photon is needed at a time to create an interference pattern, although many photons must be accumulated to visualize the pattern on a screen. Each photon contributes a single dot, and over time, these dots combine to reveal the interference pattern. This principle also applies to electrons in the double slit experiment, where reducing the current allows for one electron to pass through at a time. Despite the individual contributions, a single particle cannot create a discernible pattern on its own. Thus, while one photon or electron is sufficient for interference, a collection of them is necessary to observe the resulting pattern.