I have one EC2 instance; if I stop that instance and open a new instance, Will I be charged for it?

Hi. It doesn't matter how many instances you have running. What matters is the total number of instance hours. You can have 1 instance running for 750 hours. Or you can have have 750 instances running for 1 hour, which totals 750 hours.

Have a look at this: https://aws.amazon.com/free/?all-free-tier.sort-by=item.additionalFields.SortRank&all-free-tier.sort-order=asc&awsf.Free%20Tier%20Types=*all&awsf.Free%20Tier%20Categories=*all

Running EC2 instances comprises usage of 2+ services actually, the notable ones in question here: EC2 (compute: cpu/memory) and EBS (storage). You have to look at your aggregate usage in terms of hours and gigabytes used to determine if you're going to be charged anything.

For EC2: 12 MONTHS FREE Amazon EC2 750 Hours per month 750 hours per month of t4g.small instance dependent on region 750 hours per month of Linux, RHEL, or SLES t2.micro or t3.micro instance dependent on region 750 hours per month of Windows t2.micro or t3.micro instance dependent on region

For EBS: 12 MONTHS FREE Amazon Elastic Block Storage 30 GB: any combination of General Purpose (SSD) or Magnetic 30 GB of Amazon EBS: any combination of General Purpose (SSD) or Magnetic 2,000,000 I/Os (with EBS Magnetic) 1 GB of snapshot storage

Also, there may be a charge for IPv4 public addresses, have a look here: https://aws.amazon.com/blogs/aws/new-aws-public-ipv4-address-charge-public-ip-insights/