The frequency at which the plasma oscillation occurs is called the plasma frequency which can be calculated using this simple physics calculator based on electron number density, electronic charge, permittivity of vacuum and mass of electron.
Formula
ωp=√((nee2)/(εome))
Where,
ωp = Plasma angular Frequency
ne = Electron number density
e = Electronic charge
εo = Permittivity of vaccum
me = Mass of electron