What is the benefit of having multiple databases in the same Availability Group vs single AG per database
An Availability Group is a unit of failover. Therefore, when considering how to group databases into an AG, keep in mind that all of these DBs will fail over together.

If there are multiple databases that must failover together, then I would recommend putting those databases into a single AG. 

Similarly, you may have databases that are completely independent, where you want to put them in separate AGs so that a failover of one does not affect the other.

In some cases, you may wish to put databases into AGs based on logical groups that fail over together, even if they technically can fail over independently. 

Having too many AGs or too many DBs per AG can both be management headaches. For example: 


If you have 50 DB, each in it's own AG: you'll have 50 unique connection strings to manage, and 50 failovers to execute for a planned failover.  
If you have 50 DBs in a single AG: If there is a problem with one database that necessitates a failover, then the other 49 databases need to be failed over as well, with all 50 databases incurring a short outage during the failover.