A quorum requires that there must be 51 senators present in order for the Senate to do business, while the House requires that the majority of the membership - or 218 representatives - are present. The number required for the House of Representatives may vary if there are absences.
Often times, there will be fewer representatives or senators present on the floor than required, but a quorum will not be called unless someone requests roll. In the Senate, for example, it is assumed that there is quorum present until someone calls for roll. This is also the same for the House of Representatives. Any member may request that a quorum roll is put in place before they put something to vote.