<button onClick="number()">create</button>
<input name="number" id="number" type="text" value="1" size="3" />
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js"></script>
<script>
function number(){
var n = Number($('#number').val())
var sum = Number(n+1);
$('#number').val(sum)
}
</script>