Reference Material:

https://aws.amazon.com/blogs/compute/introducing-price-capacity-optimized-allocation-strategy-for-ec2-spot-instances/
https://docs.aws.amazon.com/whitepapers/latest/cost-optimization-leveraging-ec2-spot-instances/spot-best-practices.html

Use the price and capacity optimized allocation strategy. Allocation strategies in EC2 Auto Scaling groups help you to provision your target capacity without the need to manually look for the Spot Instance pools with spare capacity. We recommend using the price-capacity-optimized strategy because this strategy automatically provisions instances from the most-available Spot Instance pools that also have the lowest possible price. Because your Spot Instance capacity is sourced from pools with optimal capacity, this decreases the possibility that your Spot Instances are interrupted. For more information about allocation strategies, see Spot Instances in the Amazon EC2 Auto Scaling User Guide.