hieght of complete binary tree=$\left \lceil log _{2}n \right \rceil$

=$\left \lceil log _{2}63 \right \rceil$

=$\left \lceil 5.9 \right \rceil$

=6

**Alternative**

max number of nodes = $2^{(h+1)}-1$ put value $h=6$

= $2^{6}-1$

=$63$ number of node given in complete binart tree