How calculate the number of instances in Google App Engine

Question

I aim to create an application that will be deployed thanks to Google App Engine.

Before that I would like to calculate the cost of Google App Engine.

For this I

Answer 1

Completely depends on the code that runs on each request. You need to make it more specific to get a better answer. Ive had carefully implemented apps that receives thousands of requests per minute with a single instance. How long each request takes and the memory it consumes is key as well as the frontend instance type you select to use.
Appengine also lets you calibrate some parameters like min/max latency to control when more instances are launched.