Capacity Planning for SharePoint internet Site...

The "official" guide is available at this URL : https://technet2.microsoft.com/Office/en-us/library... 

under the name "Estimate performance and capacity requirements for Internet environments (Office SharePoint Server)"

You stil have access to other capacity planning guides from here :

Estimate performance and capacity requirements (Office SharePoint Server) 

In order to provide specific guidance, several configurations were tested that cover the most common usage scenarios for Microsoft Office SharePoint Server 2007 and Microsoft Windows SharePoint Services 3.0. These articles include:

• Estimate performance and capacity requirements for Windows SharePoint Services collaboration environments (Office SharePoint Server) addresses test results and recommendations specific to Microsoft Office SharePoint Server 2007 collaboration environments.

• Estimate performance and capacity requirements for portal collaboration environments addresses test results and recommendations specific to Office SharePoint Server 2007 portal collaboration environments.

• Estimate performance and capacity requirements for search environments addresses test results and recommendations specific to Office SharePoint Server 2007 environments in which the Search service is a principal feature.

• Determine resource requirements to support Excel Services addresses test results and recommendations specific to Office SharePoint Server 2007 environments in which Excel Services is a principal feature.

Each of these environment-specific articles provides the following information:

• Key characteristics General characteristics of the scenario, such as system security, varieties of common user operations, and how the system will grow over time. Assumptions for system architecture, user authentication methods, user response times, and concurrency.

• Test environment The design parameters used for testing, including number of users, user operation mix, number of sites, views, indexing, and number of documents per library.

• Usage profile Details about the percentage of system resources consumed by user operations, throughput data, and the impact of scale increases on system performance.

• Recommendations Recommendations on hardware requirements, throughput, data capacity, performance monitoring, and environment-specific factors.