For a law to pass how many states have to agree?

For a law to be passed in Congress it needs majority support of the House of Representatives (which is made up of 435 members from districts of similar size) and majority support in the Senate (each state is allocated two members, regardless of population so 100 Senators in total). It also needs to be signed by the President.

You may be thinking of a constitutional amendment, which does need support a certain number of states.