  In the archetypal monolayer semiconductor WSe$_2$, the distinct ordering of
spin-polarized valleys (low-energy pockets) in the conduction band allows for
studies of not only simple neutral excitons and charged excitons (i.e.,
trions), but also more complex many-body states that are predicted at higher
electron densities. We discuss magneto-optical measurements of electron-rich
WSe$_2$ monolayers, and interpret the spectral lines that emerge at high
electron doping as optical transitions of 6-body exciton states ("hexcitons")
and 8-body exciton states ("oxcitons"). These many-body states emerge when a
photoexcited electron-hole pair interacts simultaneously with multiple Fermi
seas, each having distinguishable spin and valley quantum numbers. In addition,
we identify the energies of primary and satellite optical transitions of
hexcitons in the photoluminescence spectrum.
