Auto numbering every time a fillable pdf form opens and saves a file with the new number each time

I am trying to generating automatic numbering for a file each time it opens starting from 000501 then getting it to save the form after completion on exit. I enterred the code below:

if (this.documentFileName == "TFMC Work Order Form.pdf")

if (global.count == null) //checks to see if count exists

{

global.count = 000501 //sets initial count, you can change to suite your needs

global.setPersistent("count",true) //makes the counter a global object so it is persistent data

}

else

global.count++ //increments the the counter by one when the document is opened

var f = this.getField("Order Number").value = Number(this.getField("Order Number").valueAsString)+1; //creates a variable for the form field where the counter will be displayed

f.value = global.count //sets the form field to equal the counter

but it doesn't generate the number even if I start the field area with 501, howver if I go into prepare form the number sometimes runs a counter in the field 4, or 5. Auto generating only when I go into prepare form  not always, only sometimes. 

Can you assist?