Why is AWS shutting down automatically all the instances launched after the fifth?

I cannot start more than 5 instances. Is this normal behaviour for an AWS Educate account on us-east1?

Yes, this is normal. AWS has Service Quotas for a few reasons, including protecting their service, protecting new customers against bill shock, and preventing huge bills if an account is compromised.

As accounts get older and pay their bills the limits increase automatically, or you can request a limit increase.

The AWS Service Quotas dashboard shows you your current limits, and makes it fairly easy to extend those limits.

Here's the dashboard for standard instance for my account, which is probably at least five years old. We can see the default limit is 5, but because my account is old and fairly well trusted I can launch 1848 instances. I'd actually rather it stayed at five, I could probably request that if I wanted to.