Achieving real-time updates on large scale of concurrent users using AWS

In our solution we are trying to support real-time updates on large scale(around million concurrent users) with low latency and keep low maintenance cost using AWS. Some points regarding our solution is:

1. Same data has to be broadcasted to all users at once regularly(eg: Live scores)
2. The data is only to be broadcasted one-way

We are exploring using AWS appsync or use Autoscalling along with self-managed EC2 instances maintaing socket connections to send real-time updates. What is the general practice at this scale, is there any caveats in trying to autoscale and manage at our end because we feel this approach leads to very minimum and non-variable costing from using AWS services.