var inv = 1;
$("#faPlus0").click(function () {
  if (inv < 10) {
     $(".bubbleBox").append("<span id='input-append' class='badge badge-secondary'><input type='text' name='invo" + inv + "'id='bubble' placeholder='Navn' onkeypress='this.style.width = ((this.value.length + 2) * 9) + 'px';'></span>");
                    inv++;
       };
    });

<span id="input-append" class="badge badge-secondary"><input type="text" name="invo0" id="bubble" placeholder="Navn" onkeypress="this.style.width = ((this.value.length + 2) * 9) + 'px';"></span>

.append("... onkeypress="this.style.width = ((this.value.length + 2) * 9) + 'px';"> ...