Can you use repeating numbers like $\pi$, and $e$, as the Key to OTP ciphers?
An OTP is completely broken if you use a key that can be predicted. As such, $\pi$ would be a terrible choice. The key needs to be unpredictable, nonrepeating, and completely random. $\pi$ satisfies the nonrepeating aspect, and (looks like it) satisfies the randomness attribute, but is predictable since I can simply search online for 1 million digits of $\pi$.
To clarify, any number people have actually heard of is a bad choice. $\pi$, $e$, the coefficient of rolling friction across a carpet, are all bad choices. You need to use a number that has never occurred before, and never will; aka randomly generated by you, not taken from a mathematical constant.