Yes, you create a single auto scaling group to scale instances across different purchase options. This is from the AWS Documentation:
Yes. You can provision and automatically scale EC2 capacity across different EC2 instance types, Availability Zones, and On-Demand, RIs and Spot purchase options in a single Auto Scaling Group. You have the option to define the desired split between On-Demand and Spot capacity, select which instance types work for your application, and specify preference for how EC2 Auto Scaling should distribute the ASG capacity within each purchasing model.