You can find your API key in the "Personal Details" section in Settings. The API key is unique to your account, and you can use the same API key to access data for all of your sites
The API employs a complexity-based request limiting scheme. The limit stands at an average of 4 units per second, and each API function uses a certain number of units each time it is called based on its complexity. The number of units used by each function is documented on the function's page.
We would love to hear how you're using our API as it influences our own development. If you're continuously hitting request limits then please get in touch with us and we'll see what we can do.