Number of isomeric compounds with molecular formula ${\mathrm{C}}_9{\mathrm{H}}_{10}\mathrm{O}$ which

(i) do not dissolve in $\mathrm{NaOH}$
(ii) do not dissolve in $\mathrm{HCl}$.

(iii) do not give orange precipitate with $2,4$ - DNP

(iv) on hydrogenation give identical compound with molecular formula ${\mathrm{C}}_9{\mathrm{H}}_{12}\mathrm{O}$ is

No reaction with strong base or acid indicates presence of strong neutrality or non-reactive functional hroup with a single oxygen. No positive 2,4-DNP test indicates absence of carbonyl compound. Same compound formation upon hydrogenation indicates single double bond. Thus, it is an ether.

As per the language of given question, the best possible isomeric structure is $\mathrm{Ph}-\mathrm{CH}=\mathrm{CH}-\mathrm{O}-{\mathrm{CH}}_3$ (cis and trans). So, the answer is $2$.