You use MySQL, not Microsoft SQL Server. Consequently, those square brackets cannot be used for escaping field names. In MySQL, you use backticks
`
instead. But anyway, you do not need to escape the field name here, it is not a reserved word nor does it contain spaces.
MySqlCommand cmd = new MySqlCommand("SELECT COUNT(*) FROM tbl_register WHERE (Email_Id = @Email_Id)", con);
Also, make sure you have an index on
Email_Id
.