Text inputs can accept numbers but if you want to be sure that the user puts in a number and not text you can use a number type instead of text.

Paste these four lines just before the submit button:

    <label for="idnumber">Your ID number: </label>
    <input type="number" name="id" id="idnumber">

Save and try sending a number.  It should work.  Try putting some text in the ID number field and it should fail.  If you are using an old browser it might still work.

You can also set a minimum and maximum allowed value for the number by adding those attributes.  Change the above new input element by adding max and min like this:

<input type="number" name="id" id="idnumber" min="1000" max="9999">

Try various numbers above, between and below those figures and see what happens when you submit the form.