Can a senator be reelected an unlimited number of times?
Under current law, yes.
Congress is divided into two houses: The House of Representatives: Members of the House of Representatives can serve for 2 years once elected, but have an unlimited number of times they are allowed to run for reelection. The Senate: Members of the Senate can serve for 6 years once elected, and also have an unlimited number of terms that they are allowed to run for again.
Yes, the Prime Minister can be reelected as an MP any number of times, even if they are no longer prime minister following their reelection. If you mean 'reelected as Prime Minister', the Prime Minister is not directly elected to begin with. However, the term of a Prime Minister is indefinite, and a Prime Minister can be reappointed by the Governor General any number of times. The only limitation on a Prime Minister's term is…
Yes. There are no term limits on members of the US House or Senate, so they can be reelected, theoretically, an unlimited number of times. A further note on this is that the US Supreme Court has ruled that any attempts by the States or Congress to impose term limits are unconstitutional. Therefore, a explicit Amendment to the Constitution will be required to impose any term limits.