What is the oxidation number of '$\mathrm{Ti}$ in the compound ${\mathrm{Na}}_2{\mathrm{Ti}}_3{\mathrm{O}}_7$ ?

Since in titanium monoxide $\left[\mathrm{TiO}\right]$, dititanium trioxide $\left[{\mathrm{Ti}}_2{\mathrm{O}}_3\right]$ and titanium dioxide $\left[{\mathrm{TiO}}_2\right],$ respectively, titanium exhibits oxidation states of +2, +3, and +4 in its compounds. The most stable oxidation state is  +4.

Hence the oxidation number of  $\mathrm{Ti}$ in the compound  ${\mathrm{Na}}_2{\mathrm{Ti}}_3{\mathrm{O}}_7$ is +4.