I think you are asking two separate questions, (1) how many stopovers can you have and (2) where can you take them.
As to the # of stop overs, you only get one (assuming multi-zone intl redemption), but you also get an open jaw. If you are clever you can use the open jaw to mimic another stopover (but it generally needs to be near your endpoint).
As to where you can take them, NW has cracked down on this in recent years. There is some wording on the website somewhere, but I think the gist of it is that it can only be a NW/DL hub (or a partner hub if you are transiting that point on a partner flight). As for "creating" a stopover while in route, you dont want to try this. I've done this on paid tickets with NW and can tell you that once you check in for the entire trip you have booked, you need special authorization to do this, and I am sure they scrutinize the fare rules to make sure you are entitled to this.