Hi Manoj,
Certain office layout has been made out providing enough work space, paraphernalia etc., to accommodate persons as planned. There could more or less no.of persons existing and on roll on a given day. Possible to keep few seats vacant, some on sites/projects/tour/ leave etc. Therefore it's right to arrive at cost per seat on the basis of seating capacity + bench strength as well. More the No.lesser the cost/seat.