Number of words in $\{pi,po\}^*$ of length at most 9
Here are all words in $L^*$ of length at most 9:
$$
\epsilon, \\
pi, po, \\
pipi, pipo, popi, popo, \\
pipipi, pipipo, pipopi, pipopo, popipi, popipo, popopi, popopo, \\
pipipipi, pipipipo, pipipopi, pipipopo, pipopipi, pipopipo, pipopopi, pipopopo, popipipi, popipipo, popipopi, popipopo, popopipi, popopipo, popopopi, popopopo.
$$
In total, there are 31 words.

More generally, suppose that we have $m$ distinct words of length $\ell$. Then the total number of words of length at most $n$ is
$$
\sum_{r=0}^{\lfloor n/\ell \rfloor} m^r =
\frac{m^{\lfloor n/\ell \rfloor+1}-1}{m-1}.
$$
In our case, we get $\frac{2^{\lfloor 9/2 \rfloor + 1}-1}{2-1} = 2^{4+1}-1 = 31$.