In general, how receptive is Amazon to service quotas increase requests?

Hello.

As you may know, instance type quotas are set low by default for AWS accounts.
https://docs.aws.amazon.com/ec2/latest/instancetypes/ec2-instance-quotas.html

When you apply to increase your EC2 instance limits, how likely is AWS to approve the request?

Quota relaxation requests are automatically approved if the quota is small, but if the quota is large, a review is performed on the AWS side.
At that time, you may be asked about your use case, etc., so if you answer accurately and are approved, you will be able to use it.
Therefore, if the use case is unclear or is not recognized as a use case by AWS, the application may be rejected.
https://docs.aws.amazon.com/servicequotas/latest/userguide/request-quota-increase.html

For adjustable quotas, you can request a quota increase. You adjust applicable quotas at the account level or the resource level. Smaller increases are automatically approved, and larger requests are submitted to AWS Support. Larger increase requests will take time to review, process, approve, and deploy. You can track your request case in the AWS Support console. Requests to increase service quotas don't receive priority support. If you have an urgent request, contact AWS Support.