Microsoft released the Capacity Calculator for Skype for Business Server 2015 to help the administrator to identity the hardware requirements.
Here is the link for download: https://www.microsoft.com/en-us/download/details.aspx?id=51196
Written by Anderson Patricio
Anderson Patricio is a Canadian MVP in Cloud and Datacenter Management, and Office Server and Services, besides the Microsoft Award he also holds a Solutions Master (MCSM) in Exchange and several other certifications. Anderson has been contributing to the Microsoft Community with articles, tutorials, blog posts, twitter, forums and book reviews. He is a regular contributor here at ITPROCentral.com, MSExchange.org, Techgenix.com and Anderson Patricio.org (Portuguese).