Personally, I think that even if e-cigs are banned in the U.S., we'll still be able to get supplies in from outside the country. It may be a bit hit or miss, and there will undoubtedly be delays. It will also likely be more expensive.
I have chosen to stock up. Truthfully, I have hoarding tendencies in the base case . . . you know you've got "issues" if you start getting nervous when there are less than 12 rolls of toilet paper in the laundry room.
I have moved to mods for a number of reasons, one of them being that batteries become a non-issue. Batteries used by mods would continue to be easily accessible if there were a ban. This allows me to stock up on more atomizers and liquid.
I hesitate to post how many atomizers or how much liquid I have because even to me it seems excessive.
And if things really become difficult, there are alternatives out there. For example, the Iolite (in no danger of being banned) allows you to vaporize tobacco, herbs, and/or liquid with no atomizers and no batteries. It runs on butane, which is easily accessible and cheap.
How much is enough? Who knows. It mostly comes down to finances and a personal comfort level.