Please recommend EC2 instance type.

There's not a simple "just do this" answer to this question because at that scale there are many moving parts that need to be considered. I can't recommend a single EC2 instance that would handle that scale - it would need to be a cluster of instances and which types and how many is going to depend on request patterns which are unknown. You're absolutely correct that a load balancer is going to be required but how that should be configured; where your static and dynamic assets are stored; how you handle authentication; and a bunch of other things all need to be considered.

At the back end you're going to have to look at which database is appropriate and how to scale that to meet traffic requirements.

You also need to look at content distribution, DDoS protection, potentially Web Application Firewall and other networking components too.

After that, things like backups, reporting....you get the idea. And we wouldn't want to forget about cost effectiveness.

Finally, your appetite and understanding of various technologies is going to guide which will be used in the solution.

For this question: Please contact your local AWS Solutions Architect team as the answers (deliberate plural) to this question will need to come from a larger discussion about your requirements.

AWS Compute Optimizer provides Amazon EC2 instance recommendations to help you improve performance, save money, or both. You can use these recommendations to decide whether to move to a new instance type.

Reference: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-instance-recommendations.html

Also, see this - some general guidance:

How do I choose the appropriate EC2 instance type for my workload?