How to Calculate the number of WFE server

Here is how you can calculate the number of web front end servers in a SharePoint farm.

Workload Characteristics	Value	Keyword
Total number of users	30000
Total number of unique users per day	20000	N
Concurrency Rate	0.3	C
Requests per day per user	400	RPD
Peak usage ratio	3
Hours in the business day	24
Average peak RPS	83.33333333
% Low-cost requests	0.25
% Medium-cost requests	0.4
% High-cost requests	0.35
Weighted peak RPS	266.6666667
Number of WFEs needed (@100 RPS/server)	2.666666667

Calculate RPS:

N: Total number of unique users ("20,000 of whom access the farm on any given day")

C: Concurrent user rate ("30 percent of the users will use the portal at the same time")

RPD: Requests per day per user ("Each user makes 400 requests in the course of a business day")

Peak usage ratio = 3 ("peak usage can be as much as three times")

Average RPS = (N * C * RPD * Peak Usage)/seconds

Calculate Weighted RPS based on type of usage

High-cost weight:             5

medium-cost weight:     3

low-cost weight:              1

% Low requests = 25% ("25 percent of requests involve tagging documents and similar activities")

% Medium requests =  40% ("40 percent of requests consist of operations such as accessing information in legacy systems")

% High requests = 35% ("everything else")

Calculate the weighted requests:

Weighted high RPS = (Average RPS* %high-cost requests * weight)

For example, high-cost requests have a weight of 5 = (83.3*0.35*5)

Weighted peak RPS = Weighted high RPS + Weighted medium RPS + Weighted low RPS

Calculate number of WFEs:

Number of WFEs is calculated using weighted peak RPS/100

Reference for this is MOC, technet. This is applicable for SharePoint 2010 and 2013.