Can I mine from more than one device? How does the network prevent fake accounts, bots, etc. from earning Pi?

No, you cannot mine from more than one device. The network has a strict rule of one account per person. Pi uses a multi-pronged strategy to ensure Pi is not mined by fake accounts. 

First, the network uses Google’s recaptcha v3 to know if a device is operated by a human or a machine. (This technology is completely invisible to most users, since they are real humans, but bots may start seeing captchas if google’s algorithm catches them). Over time, we are also developing a machine learning algorithm based on real users’ mining behavior to predict bots.
Second, Pi’s consensus algorithm requirement (security circles) make it easy to detect fake accounts. Fake accounts will not have enough real people pointing at them via security circles. At best, fake accounts would be pointing at each other with very few incoming pointers from the rest of the network. This anomaly is easy to detect with computer algorithms.
Finally, when Pi enters its 3rd and final phase – Mainnet, only accounts validated as belonging to distinct real individuals will be honored. In other words, even if all other methods missed out on some “bot” accounts, this user validation process will further eliminate them because bot owners will not be able to prove bots as real people with real names. We are currently exploring decentralized KYC processes that would be minimally intrusive.