I can't get the logic of this problem from my mind. Will you help me, pls?

I make a manual email validation for practice. So, in email validation there's a validation that states: "@ is cannot be more than one."

I'm stuck from here:

            int atValidation = 0;
        for (int i = 0; i < txtEmail.Text.Length; i++)
        {
            if (Char.IsSymbol('@'))
            {
                atValidation++;
            }
        }

I'm sorry if my question is confusing, I can only explain like this vvv
Example:
Input Email |_______|
Input Email | rich@[email protected] |
message box: '@' cannot be more than one
Input Email | [email protected] |
message box: email is valid

There are many more rules for email validation, but if all you want to do is check for a single @ sign, you could get all of the @ characters and check the count

if (txtEmail.Text.ToCharArray().Where(x => x == '@').Count() != 1)
{
     //email is invalid
}

more