are you using a "Free tier eligible" EC2 instance? and if not pls check. (for more: https://aws.amazon.com/free). For checking your free tier usage: https://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/tracking-free-tier-usage.html). furthermore, for increasing the limit, raise a case in support. they will increase it as per your requirements. and in the free tier always use t1.micro and t2.micro.
Can you also check the vCPU Limit under the EC2 Instances page. When you select EC2 service, on the left panel (top most section), you would see an option 'Limits'. When you are on Limits page, use the filter option to choose 'Running Instances', this will give you a view of eligible vCPUs by instance family, that can be used at any given time.
To make a request to increase the Limit, select the item probably this 'Running On-Demand All Standard (A, C, D, H, I, M, R, T, Z) instances' if you are looking for 't' family, use the button 'Request Limit Increase' on the top right corner, which leads you to a form to request for limit increase.
To your other questions -
- If you are within the 750 hour limit across instances, there will not be additional charges for launching multiple instances or increasing vCPU limits.
- It is possible to launch as many instances as you like within the free tier, with in the predefined limits. 3 or 4 definitely yes.
- You can use any AMI that is marked as 'Free Tier Eligible' in the section 'Application and OS Images (Amazon Machine Image)' of Instance Launch Page. This section in the new UI has 2 tabs 'Recents' and 'Quick Start'. Tab 'Quick Start' would display the 'Free Tier Eligible' label against the AMI name.
An example Free Tier AMI - Amazon Linux 2 Kernel 5.10 AMI 2.0.20220606.1 x86_64 HVM gp2 AMI ID : ami-0cff7528ff583bf9a
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