Reserved instances are not bound to any specific VM. When you start a virtual machine and there are reserved instance slots free it will charge you the reserved instance amount. If you provision more machines it will charge the standard amount.
From: http://aws.amazon.com/ec2/reserved-instances/
Easy to Use: Reserved Instances are easy to use and require no change to how you use EC2. When computing your bill, our system will automatically apply Reserved Instance rates first to minimize your costs. An instance hour will only be charged at the On-Demand rate when your total quantity of instances running that hour exceeds the number of applicable Reserved Instances you own.