When you reach the end, the series will stop, when the series stops you won’t be able to create the object with that series until you extend it.  You only have to add another valid from or change the end value.  The starting point you’ve shown would have 1 million invoices.  How many do you think you need?

You can get up to 15 digits in the End Value, I wouldn’t expect it possible to reach that value in your lifetime.

You can enter 15 digits for the End value

ex :999999999999999

Thank you for the response. The reason I was asking was a request from the business to perhaps implement a shorter number of characters and was hypothetically wondering what might happen if you reach the end. 

Ahhh, ok, reasonable question then.  There isn’t anything detrimental to shortening it really, it can be extended later.  Likewise, there is no real benefit from shortening it aside from visually.  You’ll find the actual number doesn’t matter in the overall scheme of managing finance.  It is important knowing what the one you are working on is, but doesn’t matter the length or even the actual number.