JSFiddle - React, Tailwind, and code Playground

HTML

<input id="a" class="activity_order" maxlength="2" size="2" type="text" value><br>
<input id="b" class="activity_order" maxlength="2" size="2" type="text" value><br>
<input id="c" class="activity_order" maxlength="2" size="2" type="text" value><br>
<input id="d" class="activity_order" maxlength="2" size="2" type="text" value><br>
<input id="e" class="activity_order" maxlength="2" size="2" type="text" value><br>
<input id="f" class="activity_order" maxlength="2" size="2" type="text" value><br>
<input id="g" class="activity_order" maxlength="2" size="2" type="text" value><br>
<input id="h" class="activity_order" maxlength="2" size="2" type="text" value><br>
<input id="i" class="activity_order" maxlength="2" size="2" type="text" value><br>
<input id="j" class="activity_order" maxlength="2" size="2" type="text" value><br>

JavaScript

$(".activity_order").keyup(function() {
    var self = this;
    var ary = [];   
    $(".activity_order").not(self).each(function(t) {ary.push(this.value);})
    console.log(ary.max());
   /* if(ary.max()==0){
      if (this.value != 1) {
        alert("must start with 1")  
        self.value = ""
      } 
    }else{
        next_value = ary.max() + 1;    
      if (next_value===this) {

      }else{
        alert("should be "+ next_value)
      };
    }*/
  })