The number of votes required to override a veto depends on the number of representatives present in the chamber during the vote. The current interpretation of the veto override rule is that 2/3 of all members PRESENT are needed to override a veto (though I suppose some argue that the constitution requires 2/3 of the total membership.) However, historical practice has been to require 2/3 of all present voting members as long as there is a quorum. This means that potentially an override majority could be anywhere from 146-290 members while in the Senate the necessary votes could be anywhere from 34-67 depending on how many congressman are present during the vote. (By the way these same numbers apply to proposing amendments to the constitution.)
Wrong. The 2/3 is the total of members in each House not those who are present for a particular vote.