Increase Service Quota for RDS Oracle

Your best bet is to open a ticket with AWS Support.

The service quota for RDS applies to all database engines supported under RDS. Please request service quota increase via the console. https://docs.aws.amazon.com/servicequotas/latest/userguide/request-quota-increase.html

Not that I am aware of, as the instance limit is for RDS, not Oracle specifically. The service quota page to request a database instance count increase from the default of 40 can be found here, for the us-east-1 region (replace us-east-1 with the regions where your database instances are located):

https://us-east-1.console.aws.amazon.com/servicequotas/home/services/rds/quotas

Scroll down to DB instances on the first page, click on it, and you can increase the instance count from the next page.