Shout-out for "AutoSpotting", which transparently re-launches a regular On-Demand ASG as spot instances, and will fall back to regular instances:
https://github.com/AutoSpotting/AutoSpotting/
Means that you can be reasonably certain you'll never run out of capacity unless AWS runs out of every single instance type you have requested, terminates your Spot instances, and you can't launch any more On-Demand ones.
(and even so, set a minimum percentage of On-Demand in AutoSpotting to ensure you maintain at least some capacity)
Combined with the fact that you can have an ASG with multiple instance types: https://aws.amazon.com/blogs/aws/new-ec2-auto-scaling-groups...
Means that you can be reasonably certain you'll never run out of capacity unless AWS runs out of every single instance type you have requested, terminates your Spot instances, and you can't launch any more On-Demand ones.
(and even so, set a minimum percentage of On-Demand in AutoSpotting to ensure you maintain at least some capacity)