Currently the shortcut module is configured to have a limit of 7 shortcuts per set. When you try to add an eighth, it (silently) replaces the seventh.
At http://drupal.org/node/511286#comment-2038092 (while working on a version of the initial shortcut patch that did not impose a limit) I wrote:
The rationale for not limiting the number of shortcuts is as follows:
* There is no way to detect when there are too many links to fit, so any limit would be arbitrary.
* It's unclear what we'd do in the user interface when they hit their limit and try to add another one. Any message or confirmation we put there seems like it would just get in the way.
* The current patch effectively already does put such a "message" on the screen, but in a less intrusive way. Once they add too many links, they'll notice the layout looks wrong and realize themselves that they need to remove one. Nice and simple, in my opinion.
In general, we cannot assume these shortcuts are going to be used in a particular way in the toolbar, and Drupal does not normally take the approach of limiting people based on assumptions about how they will be using a feature, so I think there might be argument for configuring the module by default to not have a limit.
Also, @alpritt at http://drupal.org/node/680500#comment-2460288 wrote this:
For the record, the way we deal with the 7 shortcut limit by arbitrarily disabling the last shortcut and replacing it with a new one is a definite WTF. It is probably too late in the dev cycle to rethink that one though. A drupal_set_message() might be worthwhile in the meantime however.