If i keep adding sulfuric acid into water, will it keep dissolving forever?
If you remove the last two or so percent of water from concentrated $\ce{H_2SO_4}$, it starts fuming, giving off SO3 until it again contains enough water to be stable.

$\ce{H_2SO_4 &lt;-&gt; SO_3 + H_2O}$

Pure sulfuric acid does not exist, so there is no clear yes/no answer to your question. Also $\ce{H_2SO_4}$ (like all other mineral acids) does not just mix with water, but strongly reacts. So it's clear there can be no miscibility gap like with water/ether or similar pairings.