How about:
CODE
<MTEntries lastn="1" category="your_category_name">
<MTEntryCategories><MTCategoryLabel> (<MTCategoryCount>)</MTEntryCategories>
</MTEntries><br /><br />
I tested that in an index template. Don't know how it'd go down in an archive situation. The only problem I can see as it is, is if you often assign mulitply categories along with the one you want to list, in which case you'd need to pop in a value for glue in the MTEntryCategories tag. I tried out a few combinations with just MTEntryCategory, but kept getting out of context errors for MTCategoryCount. :angry:
It does depend on where you want to use it, for example, if you just want to show the number of entries in each category on it's own category archive page, you'd just need to use MTArchiveCount.
Anyway, I hope that helps?