I have noticed, Slick2D will only allow a certain number of keyboard presses at the same time. Is there anyway to change this so i can have up to 6 keys being pressed at the same time
Slick is made atop LWJGL and you are encountering not a slick/lwjgl problem but a keyboard hardware problem.
http://lwjgl.org/forum/index.php?topic=4648.0
On my laptop I can press 3 keys for the most part but on my desktop I can get like 6. Some keys work together while others do not.