I'm not really sure either, but I'm taking guesses here from the information we've gotten so far.
It's said that each Greeed has 9 core medals. I've seen in Episode 2 that when Uva materialized from a ton of Cell Medals, he had Four Core Medals in him.
Also, it seems that Ankh can stay in his disembodied hand form while Eiji is using his only Core Medal. So that means he has to have another Core Medal. Also, in the opening, we see the amount of Medals seen in the series. There are two Taka (Hawk) medals. So I'm just guessing here that there are probably three copies of each core medal in a Greeed.