Nicholas, you've got some incorrect assumptions about the way these templates work. This isn't Blogger, and you're going to have to learn to work with the tool in front of you. Most of MT's functions and limits are
not in any sort of control panel as with Blogger but within the template language itself.
First of all, what type of category template are you talking about? If it's category-monthly(-weekly, or any other variation), then what you see is entirely correct. The current month's page will only show a few posts, and there should be pagination links at bottom to move onto the next category-monthly chunk's items.
If it's a plain category archive, then yes, there's a limit set on the
MTEntries tag in the
default template. It's set to "auto," which defaults to the Entry Listing Default setting. If you want some other amount, then you have all the power--and responsibility--to change it. Find this:
CODE
<MTEntries limit="auto">
and make it whatever you want.
Also, the plugin Jadespacy pointed you at works perfectly well in MT4, though I don't think it's the solution to your actual problem.