Yes, the general limit for creating on-demand instance is 20. You may place request to create more EC2 instances.
For general purpose it is limited to 20.
Auto scaling is a totally different scenario. In auto-scaling the EC2 instance will be created according to the need and also according to the required purpose. That if needs more can scale more than 20. For the case of Auto-scaling there is no limit as they are not on-demand instances, they are generally treated as a service by EC2 for the availability.