How can we use sql server count function in Asp.net c#

Luckily, it's pretty easy:

using (SqlConnection conn = new SqlConnection(connString))
{
    SqlCommand cmd = new SqlCommand("SELECT COUNT(*) FROM wherever", conn);
    conn.Open();
    int count  = (int)cmd.ExecuteScalar();
}

Obviously, you need to modify the SQL and provide connString, the usual exception handling is also missing from my sample.