Добрый день!
Как-то так:
HTML
<input type="tel" placeholder="+48 111-111-111" id="phone" name="phone" data-field="item" minlength="9" required>
<input id="but1" value="Zamówic" type="submit">
JS
phone.addEventListener('keyup', function(evt){
let length = this.value.length
if (length < 9) document.getElementById('but1').disabled = true;
else if (length == 9) document.getElementById('but1').disabled = false;
else if (length > 9) document.getElementById('but1').disabled = true;
})
https://jsfiddle.net/pavlukd/opvnemr5/ 
