This may be determined by DLC.... Currently there are three challenge mode maps available. All 3 come with the Collectors Edition (which I just ordered with 2-day shipping, bite me!), and 2 of the 3 are available with the Amazon pre-order. I do not know if the vanilla game includes any, but I would imagine that there will be more challenge mode maps made - MP is going to be a huge area of growth for re-play in this game, and so I would imagine we will have a general trickling in of new challenge maps over the next year or two.
Want to play a game of ME3MP?
Qui-Gon_Glenn on the software of which we shall not name.... add me and the enemy shall fall in chunks of crimson salsa