A Worker Entity is the individual Worker within a Worker Process that listens to a specific Task Queue.
A Worker Entity listens and polls on a single Task Queue. A Worker Entity contains both a Workflow Worker and an Activity Worker so that it may make progress of either a Workflow Execution or an Activity Execution.
Can a Worker handle more Workflow Executions than its cache size or number of supported threads?
Yes it can. However, the tradeoff is added latency.
Workers are stateless, so any Workflow Execution in a blocked state can be safely removed from a Worker. Later on, it can be resurrected on the same or different Worker when the need arises (in the form of an external event). Therefore, a single Worker can handle millions of open Workflow Executions, assuming it can handle the update rate and that a slightly higher latency is not a concern.