Can I put in any number of cell phones I want as authentication devices for Duo? Can I put in someone else’s phone?

For more information on adding more phones to your Duo Security account, click on this article.

Yes, you can add as many devices as you want. However, it is not recommended for you to add too many devices or someone else's cell phone, because this will end up in the authentication becoming a really big hassle rather than a security measure because you will have to sift through a large list to find the device that you currently have access to. Remember that you can always add or remove devices at any time, and it is better to keep a small list of devices you will be using. See here for more information.