vs virtual machines on Azure

Hi all,

I need to set up my own shiny server and I'm looking into renting VM at Azure. When renting VM I have to choose the number of CPUs and Memory.
In comparison to professional plan, how many CPU do I need and how much memory should I go with?

At the beginning it will only have to handle a traffic where 50-100 people are using it simultaneously. But going forward it will have to be able to handle thousands of users simultaneously without being slow.