I typically have one DB service per app service (not just ler cluster, unless multiple services need the same db).

Advantages:

• Simple backup/data organization, each service is self-contained
• True isolation: Unless you manually create DB accounts for each service, likely all your services have access to all data, and even with accounts there are data leaks and exploits

Disadvantages:

• You have more services running than strictly needed, but this is a minuscule impact on performance (the overhead of the DB service is typically not noticable)