If you want to carry over last month's sales, choose "renew until sold" on your listings.
However, I do not recommend this as it can make an item look "stale", especially if it is a slow seller.
Also, it creates a bit more of a challenge for inventory control purposes as I do not believe that you receive a confirmation from eBay when this happens.
Personally, I like to relist manually. Your initial quantity will be reset. However, and this important, your previous sales carry over for "Best Match" advancement purposes. You don't see this but it is there.
Bernie