The Number of threads can run simmultaneously by the server 
is equal to the number of persons can log into a web site.

Number of users loged on website is depends on Pool Size of
there connection string. By Default MaxPoolSize=100 and
MinPolSize=0.
if Max pool size in 100 so only 100 user can loged in.If 101
user will try to simultaneously you will get TimeOut error
because your connectionstring wait to free one of connection
onject out of 100.

Create a static variable and increment it every time the 
page/sit gets loaded.

Use listeners. Since you want to to know how many users have
logged in, so the login info would be stored in a session.
You can create a filter by extending HttpSessionListener,
and update a static variable when the session is created.

The value of this variable will store the number of users
currently logged on.

1. If the website is static pages no data base interaction 
then using static variable we can find out how many users 
loged into the system.
2.If the webpage is data base centric and for accessing  
any page we are retriving data then we can find the max. no 
of connections .