Octopus Deploy will not prevent you from adding a number of targets that exceed your license limit. For example, if you have a target limit of 100, you can add a 101st and 102nd target.
What will happen is that while most of the UI will continue to function, you will no longer be able to deploy or promote releases until the number of active targets is less than or equal to your limit. You can disable the new targets to bring your active count down or you can remove them.
If you need to temporarily exceed your license, for a migration of servers for example, please contact firstname.lastname@example.org so that we can provide you with a temporary license to use during that time.