To elaborate on Ming's post, Korea wasn't in a serious naval war after they beat back Hideyoshi until the end of the 19th century (strictly speaking, that was a Japanese fait accompli rather than a war, but...), and they didn't need that many at that time to trash the Japanese navy when they first started building them. On the other hand, they did have pressing issues with the Manchu tribes starting around the 17th century, where a navy would be the last thing to see use. In all effectiveness, they didn't see that much of a need for a lot of ships when the only use for their navy for almost 400 years was whacking any pirates that got a bit too active.
Despite that, even during the war against Hideyoshi, the majority of Korean ships were still panokseon, wooden two-masted warships (with oars for low-wind conditions) that also dramatically outclassed their Japanese counterparts, and Korean tactics tended to be less a matter of boarding actions and more ship-on-ship ranged combat or breaking the enemy's back via ramming...a bit of a Drake versus the Armada, for a western parallel, only the waters were more treacherous and the Japanese less able to soak up losses. The turtle ships were a very useful tool, but they were never more than a small portion of the navy even after said navy was downsized. In point, after Chilchonryang, every single turtle ship was lost, and the next battle at Myeongnyang (13 panokseon versus 133 Japanese combat-capable ships and 200-some auxillaries) saw 0% losses for Korea and the Japanese scattered. No turtle ships whatsoever were involved, just a superior navy in quality and use.