They exist as spirits who serve God and His people. No, human beings do not become angels when they die. The Scriptures are clear on this point. The Bible describes angels as having both intelligence and a will. However, angels do not know all things.
Angels are extremely powerful beings.
The power of the angels is never unlimited, but is always subject to the will and authority of God. Evil angels too are powerful beings. Angels, like God, do not inhabit the same physical dimension that human beings inhabit. From time to time, they are ordered by God to appear in our physical dimension. The Bible does not give us an exact number, but does clearly teach that there are incredibly large numbers of angels who serve God.