<input type="number" name="quanti"  id="quanti"  class="form-control"
    onInput="validarInput('<?php echo $row["idArticulo"]; ?>')"
    onKeyPress="return soloNumeros(event)"
/>

<button type="submit" name="add_to_cart" id="<?php echo $row['idArticulo']; ?>"
    class="btn btn-warning add_to_cart" >Agregar al carrito</button>

<!-- acepta solo numeros, funciona bien-->
<script  type="text/javascript">
    function soloNumeros(e) {
        var key = window.Event ? e.which : e.keyCode;
        return ((key >= 48 && key <= 57) ||(key==8))
    }
</script>

<script type="text/javascript>
function validarInput(iden) {
    /*Funcion del botón, funciona bien, pero solo cuando yo escribo*/
        document.getElementById(iden).disabled = !document.getElementById("quanti").value.length;
    }
</script>