I wish I had seen this thread earlier.
In October of last year, I left my manager-level job at Dollar General corporate headquarters in Goodlettsville, TN after almost 7 years working there. Now, I was in HR, not Store Operations, but I can confirm/correct or deny most of the information in this thread and answer questions anyone might have.
To start with, when I joined DG in early 2009, they had about 7200 stores. All DG stores are company owned, no franchises. The buildings are leased. When I left last year they had well over 12,000 stores and were planning to open 900 this year, so they are probably at or near 13,000 by now. Really phenomenal growth.
They are most densely located in the south and southeast and Midwest parts of the country, but have been aggressively expanding out west (CA/WA/OR) and in New England. In 2014/2015 they attempted to purchase, then take over, Family Dollar but lost that bid to Dollar Tree; so they have put even more focus on rapid expansion to remain the segment leader in discount retail.
What else? Contrary to what was asserted above, margins on most products are very small and they control costs by aggressively controlling costs (both in terms of merchandise and labor).
They do not sell odd-lot or overstock items - they sell regular brand names, although their selection evolves over time like any other store. They also usual focus on smaller, low-cost packages to appeal to their cost-sensitive consumer base. They are the leading retailer of many popular brands (Unilever, Kraft, Proctor & Gamble, etc.) They also push their private labels, of course, which offer both lower cost to the customer and higher margins for the company. In recent years they’ve added alcohol and cigarettes to their shelves to bring in more traffic, which has largely been successful.
I’ll go back through the thread to see if there are any other specific comments or questions I can address.