I tried the "automation Rule" many years ago when I was running 3,000 to 4,000 listings, including several hundred seven day auction format listings
It did not work for me.
The problem I saw was eBay relisting the "auctions" within a few or several hours of ending - and spread them over several hours - not immediately upon ending as is done with fixed prices GTC listings (automatically extends for 30 more days every 30 days the second it ends)
While a seller cannot control when a buyer purchases a fixed price listing, I always tried to be online when auction listings ended so I can immediately inform the buyer and send combined invoices if and when required.
With eBay relisting at different times, often several hours later, that defeated my way of doing things. eBay took control of the timing away from me (with auction closing in the middle of the night), I stopped the experiment.
I found it much easier to simply relist unsold auction listings using the "bulk" relisting function, at a time of my choosing. Also, over time, the "auction" format has lost most of its appeal with eBay buyers who now largely prefer using the fixed price format (they do not have to wait a day or seven to find out if they purchased an item). As such, I have been running a limited number of auctions in the last few years, usually in answer to one of eBay's "free listing" promotion.
To answer your second question, a new listing number is assigned by eBay when a new listing or relisting starts.
Good Luck.
