Welcome to Cayenne!
@akers.kenneth66 was able to get an arduino connected through USB here Arduino and Raspberry Together - #4 by akers.kenneth66 My only caution is that USB connections are meant for testing and are generally not going to be suited for a “production” implementation. I’ve never tried it but from what I read about it on the Blynk forums it is not recommended. I would personally recommend getting an ethernet shield.
Yes, you will need to register each Arduino to get it on the dashboard.