For event registration, if the capacity is reached, will it be impossible to register?

Yes. If a capacity limit is set, once that number is reached you will not be able to register. However, the capacity can be changed at any time, so if the venue has room you may increase it partway through. If you decrease the capacity, you can set it lower than the number of people who have already registered, but in that case those who have already registered will not be canceled — the event will simply remain full.