Aneet is correct that traffic distribution is a function of Load Balancer either a hardware or software one. It depends on the physical architecture of your site and how WAS-AEM relationship is set up.
Dispatcher (a type of software LB) is also responsible to load-balancing the requests to multiple publish servers in a farm. Check
The distribution of traffic would be regulated by the Load Balancer sitting infront of these AEM instances. Once the request comes through to an AEM instance, it would respond back (granted the resource has anonymous access). The only restriction from a Product perspective can be at the dispatcher level, but I am assuming that when you mention traffic to AEM instances, you're including dispatcher to be a part of it.