How do I determine internet bandwidth that is allowed for EC2 instance ?

You need to differentiate between multi-flow and single-flow traffic. For single-flow traffic or multi-flow traffic in instances with <32 vCPUs you generally have up to 5 Gbps. If the instance has at least 32 vCPUs, for multi-flow traffic to an internet gateway (and therefore to the Internet) you can utilize up to half of the network bandwidth available to the instance. For example an m5.16xlarge can have up to 10 Gbps for multi-flow traffic to the Internet. You can read more here.