Find the number of mixed oxides among the following:

$\begin{array}{r}{\mathrm{Mn}}_3{\mathrm{O}}_4,{\mathrm{Fe}}_3{\mathrm{O}}_4,{\mathrm{Co}}_3{\mathrm{O}}_4,{\mathrm{Mn}}_2{\mathrm{O}}_3,{\mathrm{MnO}}_2,{\mathrm{Fe}}_2{\mathrm{O}}_3,\mathrm{NiO}\text{, }{\mathrm{CrO}}_2,{\mathrm{CrO}}_3,\\ \end{array}$

${\mathrm{Mn}}_3{\mathrm{O}}_4,{\mathrm{Fe}}_3{\mathrm{O}}_4,{\mathrm{Co}}_3{\mathrm{O}}_4$ are mixed oxides .The chemical substance known as manganese(II,III) oxide has the formula ${\mathrm{Mn}}_3{\mathrm{O}}_4$. The chemical formula for manganese, which exists in two oxidation states (+2 and +3), is sometimes expressed as $\mathrm{MnO}-{\mathrm{Mn}}_2{\mathrm{O}}_3$.
Response. Two distinct oxides are combined to form mixed oxide. For instance: ${\mathrm{Fe}}_2{\mathrm{O}}_3$ and$\mathrm{FeO}$ combine to form ${\mathrm{Fe}}_3{\mathrm{O}}_{4_{ }}$.
An inorganic chemical with the formula ${\mathrm{Co}}_3{\mathrm{O}}_4$ is cobalt(II,III) oxide. It is one of two cobalt oxides with good characterization.