It is $\bf{23}.$
$(p-1)(\text{key_ptr_size} + \text{record_ptr_size}) + p. (\text{block_ptr_size}) \leq 512$
$\implies (p-1) (10+ 8) + p \times 5 \leq 512$
$\implies 23p \leq 530$
$\implies p \leq 23.04$
So, maximum value of $p$ possible will be $23.$