aws - how to calculate how many instances I need?

My app was easily deployed with OpsWorks.

Currently it has just 100 daily access in a single page with a single MySQL insertion.

HaProxy - 1 instance t2.micro
App     - 1 instance t2.micro
RDS     - 1 instance t2.micro

Tomorrow I'll get 50k access in the same minutes (facebook posting and ads).

My question is: how many instances and servers do I need to this traffic? Maybe something like:

HaProxy - 10 instance t2.micro
App     - 10 instance t2.micro
RDS     - 1  instance t2.large

Is this enought? How to know this?