Honest hearts in pretty much confirmed in-game by game files saying "Honestheartsgrafiti" much like the Dead Money grafitti.
1) Five "questing marks", one achieved from Dead Money.
2) Fallout 3 had five DLCs.
The questing marks however in the regular game also go up to five before you get an experience bonus after completing five quests.
Also, Dead Money was set outside of the map. That doesn't mean they're going to use the blocked off regions of the vanilla game for future DLC. That may have been blocked off for various other reasons.
Now then, that doesn't mean I don't think there'll be five DLCs (I'm sure there'll be five). But you shouldn't be saying it like it's a fact ("There is five or six DLCs being made for New Vegas."), since it's still purely speculation.
