Unless they are certified, drones do not need to be registered, but you, as drone operator/owner, must register yourself. You do so with the National Aviation Authority of the EU country you residence in. (https://www.easa.europa.eu/domains/civil-drones/naa)
You register once, independently of how many drones you have operating in the ’open’ or the ’specific’ category. Your registration will be valid for a period defined by your National Aviation Authority, after which you need to renew it.
However, you do not need to register yourself if your drone(s):
- weighs less than 250g and has no camera or other sensor able to detect personal data; or
- even with a camera or other sensor, weighs less than 250g, but is a toy (this means that its documentation shows that it complies with ‘toy’ Directive 2009/48/EC);
A drone is certified when it has a certificate of airworthiness (or a restricted certificate of airworthiness) issued by the National Aviation Authority. In this case, it requires a registration. A certified drone is needed only when the risk of the operation requires it. So certification is never needed for drones operated in the ’open’ category.
Relevant regulation: article 21 of EU regulation 2019/947.