Hi @ptitkoko,
Please note that, by default, our PDF invoices plugin generate sequential numbers, without gaps. However, the invoice numbers are assigned based on when the document is generated, and this does not always match the sequence in which orders arrive (unless you generate an invoice at order creation, regardless of whether you have received payment or not, e.g. by sending the document to the administrator).
For instance, let’s say that your next invoice number is I-101 and you only generate an invoice when the order changes to Completed status (that in fact appear to be your case). If you receive three orders in a row, with the order numbers 100, 101 and 102, but the order status changes to Completed in a difference sequence: 102, 100, 101, the invoice numbers would look like this:
Order: 100 = Invoice: I-102 (second to change to Completed)
Order: 101 = Invoice: I-103 (third to change to Completed)
Order: 102 = Invoice: I-101 (first to change to Completed)
In brief, it is likely that the invoice numbers between 2008 and 2012 were assigned to previous orders, that are maybe in the second page in your order list.
That said, you can see the complete list of your generated invoices, date of creation, and the order number they are linked, using our Number Tools extension, which you can download here: Download Number Tools (Latest release)
After, installing and activating the Number Tools extension, go to WooCommerce > PDF Invoices > Number Tools > Choose a number store, select Invoice > Click on View, and now you’ll see the complete list of generated invoice numbers.
Hope it helps you to understand what happened with the invoice number you are not seeing!