What is the best practice for defining Akka Actor Count

I am trying to figure out how Akka works and what are the best practices for using Actor Model.

I have couple of questions regarding the same.

Questions:

1. What are the deciding factors that we should keep in mind when configuring total number of Actors and Threads for below mentioned scenario?

   Scenarios:

   a. only tell is being invoked on actor (Fire and Forget).

   b. ask is being invoked (Futures and Promise).

2. What are the advantage/disadvantage of using Router e.g RoundRobinRouter(X) over manual actors creation.

3. How dispatcher orchestrates MailBox, Actor and Threads for message processing.