Tax Questions - number taxpayers, tax distribution, etc...?

Tax Questions - number taxpayers, tax distribution, etc...?
I'm looking for a bunch of tax statistics - if you have the answer, please provide the source of the data as well (the more recent the data, the better): How many taxpayers are there? (# of taxpayers, not the number of tax returns filed) What percentage of the entire federal budget is funded by individual taxpayers through federal taxes, etc? On average, what amount of an individual's taxes is designated towards Social Security? Medicare? Also, somewhat unrelated - in the federal budget, there are a large number of negative values. Do individuals contribute in any way to these through taxes? Thank you.