I'm working on a project that involves running a lot of spot instances over a short period of time, then terminating the spot reservation and starting the instances with a different AMI. If I reserve 500 spot EBS-backed EC2 instances for a day and they all have 8GB hard drive, am I paying for 500 EBS-days or 500 EBS-months (multiplied by 8GB)? If the latter, will terminating the instances and asking for new ones mean that I'm paying for another 500 EBS-months even if I use the machines just for one day?
Neither, really. EBS bills by hours. https://aws.amazon.com/ebs/pricing/
Volume storage for General Purpose SSD (gp2) volumes is charged by the amount you provision in GB per month, prorated to the hour, until you release the storage.