Question: How can the external CA limit of 5 be increased?
Answer: The max is controlled by the advanced config: 'Core.ExternalCaAuth.MaxExternalCa'. This defaults is 5; for on-prem instances, customers can go to advanced config and set this to a higher value.
Note that the more certificates added, the more resources/impact on IIS. It is best practice to keep certificates added to a minimum. There's no hard limit; performance impact will be based on how powerful the hardware hosting the service is.