For the buffer solution containing $N{H}_{4}OH$ and $N{H}_{4}Cl$, pH of the buffer solution can be increased by

On Equilibrium,  $pOH=p{K}_b+\log \left(\frac{Salt}{Base}\right)$

                                          $pH=14-pOH-\log \left(\frac{Salt}{Base}\right)$

Therefore, $pH\propto \left[Base\right]$     and       $pH\propto \frac{1}{\left[Salt\right]}$   

On adding more$N{H}_{4}OH$ pH of solution is increase similarly on adding more $N{H}_{4}Cl$