Right now, you have the option of Free Tier which provides you with a t2.micro instance free for 750 hours/month (for all instances put together). AWS does not count the number instances, it takes the total number of hours used by all those instances. So, once your instances cross the 750-hour mark, you will start getting charged.
Also, there is no charge when the instances are stopped.
For additional information - https://aws.amazon.com/ec2/pricing/