Do I need 1 RDS Cluster for each DB or a single RDS Cluster for all my DBs?

Simple answer: it depends. :)

It depends on how inter-dependent the databases are. If each is used independently by its service or microservice and you have some automation to provision the microservice then it's usually advisable to compartmentalise the whole microservice (compute + database + storage) and have it completely independent from the other microservices. It fits better the DevOps model where the Devs are not only responsible for the code deployment but also for the database and storage.

But in real world and/or with monolithic apps and/or where there is only 1 Dev team it's often the case that the database is shared. It's kind of the old on-prem mindset where a company was paying huge money for a central Oracle RAC that and for an army of DBAs who kept the lights on.

There's no right answer - it's more "modern" to have separate DBs but if the extra cost and complexity doesn't warrant it then by all means use a shared DB. And as Tim says - benchmark the performance.