All counts take the following additional (undocumented) attributes:
singular="1 sometext" plural="# sometext" none="No sometext"
Where sometext is whatever you want.
Thus, you could do:
<mt:archivecount singular="1 entry" plural="# entries" none="No entries">
and it will say "1 entry", "2 entries" (or whatever number of entries it finds) or "No entries"
Reported on Movable Type 4.3
For whatever it's worth.
My bad. Missed that. I wonder why. A tag attribute in the design section. Hm. Kind of makes sense except it doesn't. This MT documentation can be so darned frustrating ...