ff XII beastiary

[jonc2006]

i have looked through the guide to the game but i cant seem to find this out, when you go into your clan primer and go to the beastiary section, ive noticed that on some of the monsters ive fought on the list, there is a number followed by the word "needed" for example: "monster a" needed XX (XX being a number). what does this mean? its on several of them, not all, so i dont have a clue what it means by "needed" and what it is "needed" for.