Meaning of 我知道的就这么多了
就 has many meanings, but here it means &quot;only&quot;.
Definition + examples from 《现代汉语规范词典》:

限定范围，相当于&quot;只&quot;，&quot;仅&quot;
屋里就剩下我一个人
这次聚会就他没有来

Definition + example from《现代汉语词典》:

仅仅；只：以前就他一个人知道，现在大家都知道了）.

So it is used for a restricted scope and have the same meaning as 只 and 仅仅 (both meaning only).
Some examples where 就 has the same meaning:

家里就是他一个人 (jia1 li3 jiu4 shi4 ta1 yi2 ge ren2): In his family there was only him
三本书我看完了两本，就一本书没看了 (san1 ben3 shu1 wo3 kan4 wan2 le liang3 ben3, jiu4 yi4 ben3 shu1 mei2 kan4 le): I have read two of the three books. There is only one left that I haven't finished reading.
就去过一次 (jiu4 qu4 guo yi2 ci): only been there once
昨天就他没有来，别的人都来了 (zuo2 tian1 jiu4 ta1 mei2 you3 lai2, bie2 de ren2 dou1 lai2 le): yesterday only he didn't come, the others all came

So I would translate your sentence as
I only know this much