When both houses of Congress agree on a law, they send the final bill to the President for it to be signed into law. If the President doesn't agree, then s/he has the option to veto the bill. After this point, the Congress has the option to override the veto if 2/3 of both houses agree to override.
This means that:
At least 67 Senators must agree to the override (note how 2/3 of 100 is 66.67 approximately, so 67 is the smallest number that clears this threshold)
At least 290 House of Representative members must agree to the override. This figure is due to (2/3)*435 = 290. There are currently 435 house members.
Both of those conditions listed above must be met for a veto override to occur. This is extremely difficult and rare considering the polarizing political climate. On things that nearly everyone agrees about, the President would likely not veto the bill (since the President is likely to agree with the Congress on such issues), and a veto override wouldn't even need to be considered.
