To translate directly,
"What I know is really this much."
which can be taken to imply that the person's knowledge is limited and can only help you this far. And hence, "beyond this, I know nothing".
To put some context to the sentence above:
A: "这题目很难，我不会做。" (This question is difficult, I don't know how to do.)
B: "是真的？" (Really?)
A: "我知道的就这么多了。你还是找别人帮忙吧。" (What I know is really this much. You better
find someone else to help you.)
"就" is the shortened form of "就是" and is spoken to emphasize that "it is really like this". I think the confusion here has to do more with the understanding of the phrase "这么多" which means "this much" and not "too much" which is "太多".
For the record, "I know too much." is translated as: