What happens if I am filing taxes as single and 2 under federal and 2 under NY but i don't have any dependents?

In this case you will most likely end up owing more taxes when you file your return at the end of the year. You withholding allowances claimed during the year only effect the amount of taxes that are withheld and applied to your tax return at the end of the year. It is to your benefit to pay in more during the year so that you don't end up owing a good deal of taxes when you file your return.