There is an option to make an additional gift to the federal government, I don't know where it is in Turbotax, though. Maybe under "other tax situations."
There's no way to know how many people volunteer, the IRS would have to release that information (in anonymous form, of course.) Turbotax can't release that information even anonymously, due to taxpayer privacy laws.
(In fact, I don't see it in the instructions at all, maybe it's not on the tax forms. But you can make a voluntary payment to reduce the national debt directly to the US Treasury, the year by year stats are here. https://www.treasurydirect.gov/govt/reports/pd/gift/gift.htm)
