I am reviewing requirements for SharePoint 2013 Enterprise (https://technet.microsoft.com/en-us/library/Cc262485.aspx). Do these requirements vary based on the number of users? I am looking to size an environment that will support 13,000 users.

Thank you.

Refer to this link https://technet.microsoft.com/en-us/library/Ff758645.aspx

You need to perform capacity planning. It also comes down to what services you want to configure and availability of those services to users.

HI,Yes the HW requirements is purely depends on the number of users who is accessing and the data\storage which is going to be created.HW requirements vary on basis of above.Please check the requirements as below to configure the sharepoint.If you need more performance and stable system then you need to look for better HW.

http://expertsharepoint.blogspot.my/2014/07/hardware-and-software-requirements-for.html