Greater the number of alkyl groups attached to a positively charged carbon atom, the greater is the hyper conjugation interaction and stabilisation of the cation. Thus, we have the following relative stability of carbocation:

$$\begin{gathered} ({\mathrm{CH}}_3{)}_3{\mathrm{C}}^{+} > ({\mathrm{CH}}_3{)}_2{\mathrm{CH}}^{+} > {\mathrm{CH}}_3{{\mathrm{CH}}_2}^{+} > {\mathrm{CH}}_3^{+} \\ 9 \mathrm{hyperconjugative} \mathrm{structure} 6 \mathrm{hyperconjugative} \mathrm{structure} 3 \mathrm{hyperconjugative} \mathrm{structure} \end{gathered}$$