Register counter with maximum registration limit in ASP.Net

You need to use Global.asax file.use Application_Start

void Application_Start(object sender, EventArgs e) 
{
    // write code to insert into Database
}

also need to check if user visit count has exceeds 20 count inside Application_BeginRequest

void Application_BeginRequest(object sender, EventArgs e)
{
    //get user vist count here
    // check if count is greter than 20 then simply log out the user
}

I hope this will help you out.