Amazon EC2 Auto Scaling now supports using up to 40 instance types with mixed instances policies. This doubles the number of instance types you can specify. Previously, you could only specify 20 instance types. Now, you can specify up to 40 instance types when you first create an Auto Scaling group, and when you update existing Auto Scaling groups. You can use the Overrides section in your mixed instances policy structure to input up to 40 instance types that Amazon EC2 Auto Scaling can use to provision your On-Demand and Spot Instances. By using a variety of instance types, you can maintain your workload’s availability as you can provision your application’s resources across more Spot instance pools per Availability Zone and reduce the chance of getting insufficient capacity error in case of On-Demand. Running your application’s resources across diverse Spot instance pools also allows you to further reduce your operating costs over time.
Source:: Amazon AWS