Yes, this makes me a huge nerd, but I was very bored at an overnight...

If you search the FAA aircraft registration database using the pattern by which F9 numbers aircraft (N2xxFR for 320's, N9xxFR for 319's), you'll find 9 N-numbers (N701FR through N712FR, with a few skips for previously existing registrations). These 9 have been reserved since 11/24/14, which is the same week that the 321 order was announced. This leads me to believe that these will be the 9 321's. However, there aren't any more N7xxFR numbers reserved beyond that.

Based on this [self-disclaimer] completely speculatory and way out there "research," I would say that we're only getting the 9 for now . But we can always hope for more!

EDIT: There are also 8 more 320 registrations reserved. Based on the May bid packets, N229FR will be online in the first half of the month. That leaves 7 more to go (something like N230FR through N240FR). Maybe those are coming on as new/replacement aircraft in the near future? Or maybe they've been reserved just in case? Let the speculation begin!