AP access (landing page) is not easily restricted, as you're just visiting a web page (with or without thumbnails). Everybody can do that.
AP Documents can be opened by the same user only once on multiple different platforms. Earlier sessions will be disconnected. You can recover the previous session by simply refreshing the page, but that will disconnect the last session.
On a single platform, a user can open the AP multiple times without any restriction that I know of (std configuration).
To limit the number of simultaneous sessions a user can have, you can create a custom landing page/login page that checks sessions and accounts and refuses to open a new one if a certain limit has been reached. Somebody has already done that before. Search the community, you'll find something.