SQL Failover Clustering with Availability Groups: Single or Multiple Databases
Are arrangements below, only possible if 1 database is on the server, or could I have multiple databases below in arrangement?


You can have as many databases as your hardware will support which depends on workload, server size, and other general infrastructure items such as latency and bandwidth of the connections.

I've witnessed AGs that are ~20TB in size across 4 databases, AGs that had over 1,100 databases, and all values in between. Completely depends on your workload, hardware, and what you'd like to accomplish.