JR offers 7, 14 and 21 day passes only at this time. I’m affraid you wont be able to make any customization to the amount of days.
If you do travel around much during your entire stay, a 14 days pass is well worth it’s price since it’s much cheaper than 2x 7 days passes.
Or if you plan to stay in a city for most of your stay and not travel with Shinkansen, a 7 days pass may be fitting for your stay and be activated first when you need it.
With kind regards, Jonathan H.