1) L'Chatchilah, the volume of boiling water needed in the pot  to ensure a proper koshering is sixty times the volume of all the utensils that will be koshered in the pot [even if the utensils haven't been used in over 24 hours].

Thus, it is best to kosher one utensil at a time rather than placing them all in at once, as by doing so there is a better chance that the volume of water will be sufficient L'Chatchilah.  (See Mishna Berura Siman 452:13. See also Tur Yoreh Deah Siman 221 and Shu"t HaRashba Vol. 1 Siman 262)

If the pot being used for koshering does not hold sixty times the volume of the utensil that will be koshered in it, it is imperative that the utensils being koshered not be used for 24 hours, otherwise the koshering  is not good even B'dieved. (See Shulchan Aruch Orach Chaim Siman 452:2 and Yoreh Deah  121:2 and Chayei Adam 125:12)

2) In cases of great necessity, when the utensils are needed and less than 24 hours have passed  since they were used and they cannot be koshered in an adequate volume of water, a Rav should be consulted as to if and how  the water [with less than 60 times the volume] can be made Halachically viable for koshering. (See Shulchan Aruch Yoreh Deah Siman 95:4 and Shach and Taz there. See Shu"t Chasam Sofer Orach Chaim Siman 120, and Shu"t Tzemach Tzedek Siman 91. See also Chazon Ish Yoreh Deah Siman 23 and Orach Chaim Siman 122. See also Igros Moshe Yoreh Deah Vol. 2 Siman 34)