There are good reasons to increase the # of doses. The vaccine is relatively cheap and it probably doesn't double the cost to double the # of doses.
First off, not all people "take" (i.e. get immunity) with the first dose. If the vaccine is anything like the old ones we had (pre 1970), then at least some people are going to have to get revaccinated.
Second, there's always a chance of losing some stock due to accident, power failure, sabotage, etc.
Finally, having excess stock of vaccine can serve as a powerful inducement to either the offending country to surrender, or to other countries to join us if they themselves do not have the vaccine. We can also use excess vaccine stock to supply our allies (Great Britain? Turkey?)
Keep in mind that any smallpox bioattack will hurt the 3rd world/Muslim countries FAR worse than it will hurt us, even without immunization. It's also possible that those of us who were immunized prior to 1970 might have some residual immunity - which many in the 3rd world do not have. The thinking here might be "Better safe than sorry."
It has to be done as soon as possible. If we were attacked with smallpox tomorrow, we'd be in deep doo-doo. The vaccine won't be available for quite some time, but it's better to get the process going now rather than waiting. The sooner we have vaccine doses onhand, the sooner we can stop any epidemic in its tracks (vaccination still works even post-exposure).