Among the following oxides of 3 d elements, the number of mixed oxides are $\_\_\_\_$ $\mathrm{Ti}_2 \mathrm{O}_3, \mathrm{~V}_2 \mathrm{O}_4, \mathrm{Cr}_2 \mathrm{O}_3, \mathrm{Mn}_3 \mathrm{O}_4, \mathrm{Fe}_3 \mathrm{O}_4, \mathrm{Fe}_2 \mathrm{O}_3, \mathrm{Co}_3 \mathrm{O}_4$
