I would like to point out these two things

1. <input type="number"> elements are not really made for "selecting" values. Although you can of course use some autocomplete library to help you with that. A <select> element is probably the more natural choice for the task.
2. The variable you desire can be easily set up. In fact I would suggest to create a const instead:

const tt=document.querySelector("input");

// the "variable" you require is then tt.value:

console.log(tt.value)

In a "real" application you should probably use a more specific CSS selector, like a class or an identity.

You can try and Change :

<input maxlength="1" type="number" min="1" max="4" id ="c0r0"></input>

To :

<input maxlength="1" type="number" min="1" max="4" id="c0r0" />

Below are the two simples ways of doing this.

Method 1:

Attach a change event listener to the input element i.e. a function that's called every time the value changes. Then use the target property of the event to get the value. For more details read this onchange event

Method 2

Get the html element reference using the getElementById (read more) and then use the that element to access the value.

Note: Using getElementById is easy and simple as you get the target element while other methods return a collection and you will have to find the element of concern in the collection.

Below is a small demo of the above recommended methods

function getValue(event) {
  // Method 1
  var val = event.target.value
  console.log('first box:' + val);
}

function method2() {
  // Method 2
  val = document.getElementById('c1r1').value;
  console.log('second box:' + val)
}

<input maxlength="1" type="number" min="1" max="4" id="c0r0" onchange="getValue(event)" />

<input maxlength="1" type="number" min="1" max="4" id="c1r1" onchange="method2(event)" />