Buttons with JavaScript do not work in the form

My problem is that I have a form in my HTML that I write to the database after filling it out.

The form consists of 7 labels with 7 input fields type=’number’
It all works so far, but it annoys me because it is designed for mobile phones that I have to keep clicking on the field and entering the number.

Now I have come up with a system that I can use between the input fields type=’text’,
I have one button each for increase and decrease. I intercept this with a JavaScript and let the will be manipulated to correspond.

This only worked to a limited extent. If I comment out the “Form Tag” it works. But as soon as I comment on the “form tag” again, nothing works anymore.

Now the question is. Where is the mistake or why it no longer works in this form?

HTML excerpt:

<form action="" method="post">
    <label>Test1</label><br>
    <input type="button" onclick="dec('number')" id="dec" value="-"/>
    <input type="text" name="test1" id="number" value="0" min="0" size="3" />
    <input type="button" onclick="inc('number')" id="inc" value="+" />
  </form>

JS excerpt:

function inc(number) {
  document.getElementById("number").value++;
}

function dec(number) {
  if (document.getElementById("number").value != 0)
    document.getElementById("number").value--;
}