How many instances we can create in a single go?

as usual with AWS — it depends.  The default EC2 instance limit per account per region is 20.  The EC2 limit is really based on vCPUs.  You can look at service limits in your account console at: https://console.aws.amazon.com/servicequotas/#!/services/ec2/quotas. You can also look at the EC2 FAQ page for more info: https://aws.amazon.com/ec2/faqs/

The above is true, you can launch any number of instances in one go till you have the required vCPU as mentioned above. Here it depends on vCPU limit or quota implied on AWS account, if it is default 20 then 20 instance can be launched in one go or if it is 1000 then 1000.