In the final system there will be multiple (50 or more) stand-alone lights that need to blink synchronised.
First off I would question the use of so many Arduinos it is rarely needed and is a common beginners error.
If you do have multiple Arduinos then they has to be some method of communication with them all, you can not depend on then keeping in synchronicity for long.
If they are in separate locations then why do they need to be synchronized anyway?