What is the maximum concurrent requests and connections can ALB handle?

I dont think you can find a specific number for this question, remember it is a managed service and under the hood is not just one physical device, and it will basically scale automatically to meet varying levels of application load. The best approach always is to test and simulate.

The main reason for number of max connection handled by ALB is not document, is that, ALB is designed to scale up as and when the traffic on the load balancer will increase. If there is a gradual increase in load (i.e less than 50% increase in a 5 min window), then the ALB scales itself to higher capacity nodes and if required increase the number of node commissioned for your ALB to provide enough capacity to handle the load.

If spike in traffic is expected (i.e more than 50% increase in a 5 min window) you can always request a prewarm of your load balancer. ( usually needed if you are expecting a spike due to marketing/sale event)