${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}+2}\mathrm{O}$ is the general formula of ethers.

 To exhibit the functional group isomerism ‘n’ must be minimum.

Simple formula of ether is ${\mathrm{CH}}_3-\mathrm{O}-{\mathrm{CH}}_3$. So, the value of n is 2 where we have two isomers:

$$\begin{gathered} {\mathrm{C}}_2{\mathrm{H}}_{2\times 2+2}\mathrm{O}={\mathrm{C}}_2{\mathrm{H}}_6\mathrm{O} \left(\mathrm{n}=2\right) \\ \mathrm{Isomers}: {\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}, {\mathrm{CH}}_3-\mathrm{O}-{\mathrm{CH}}_3 \end{gathered}$$