Each function instance will be limited to a memory of 1.5 GB by the host of the function app, a group-like semantic for multiple function instances. All functions within a function app share the same resources.
A function app holds a maximum of 200 instances of a function at the same time. But there is no concurrency limitation. In practice, a function instance can accept one or more requests.
Each event trigger, for example, Azure Service Bus has its own heuristic way to scale the underlying function.