Working in Acrobat pro have a fillable PDF where I want one field set to auto populate

The best approach is to use a Validate script on the "OrderNumber 1" field. Then make all the other OrderNumber fields read only. 

// Custom Validate script
if(Number.isInteger(Number(event.value))
{
   var nInc = Number(event.value);
   for(var i=2;i<=nRows;i++)
   {
       this.getField("OrderNumber " + i).value = ++nInc;
   } 
}
else
{
   event.value = "";
   app.alert("Entered Value must be an Integer");
}