Adding the bot
Discord only lets you add a bot where you have the Manage Server permission, so that is the short answer to which of your servers you can add it to.
The invite link asks for the exact permissions the bot's features need: reading and sending messages, embedding links, managing messages for the purge command, connecting and speaking in voice channels, timing members out, and managing roles. Discord shows you the full list on the authorisation screen before you accept.
There is also a second link that asks for Administrator instead. It is there for convenience: with it you never hit a missing-permission wall later. It also means the bot can do anything on your server, so pick the first one unless you have a reason not to.