[EVA] How many souls can their be in an Eva?

[nerv at telstra.easymail.com.au]

I was just wondering how many souls an eva could hold, more specifically unit 00. I was just thinking this because I always thought it was 1, then I noticed something. In the episode with the cross compatibility test, and unit 00 goes berko with shinji inside, Ritsuko cries "is it trying to absorb shinji", or something to that effect. Now if an Eva could only hold 1 soul, their would be no chance of that, assuming unit 00 actually has a soul. If not then an Eva doesn't need a soul to work, this goes against something else, but I cannot remember what, all well. 
Sorry, my gramma isn't all that good.