i am traveling to Brazil. How much cash am i allowed to take on the plane?

I don't think there is an actual limit, but I do think that if you have more then $25,000, they have the right to investigate why you have it, where you're going, and stuff like that.

There are no limits, but you are supposed to declare anything over $10,000 when leaving the US and will need to fill out a form. When entering Brazil, you are asked to declare anything over R$10,000 or foreign currency equivalent. These laws are basically designed to prevent money laundering, if your money was legitimately earned and taxes were paid, there's nothing to worry about.

i think you can carry cash according to your needs, but you have not to tell every one that you are carrying so much cash with you. bad and evils are every corner of the world and not only in Brazil.