<$mt:ArchiveCount$>
This tag will potentially return two different values depending upon the context in which it is invoked.
If invoked within Categories this tag will behave as if it was an alias to CategoryCount.
Otherwise it will return the number corresponding to the number of entries currently in context. For example within any Entries context, this tag will return the number of entries that that Entries tag corresponds to.
Example:
<mt:Categories>
There are <$mt:ArchiveCount$> entries in the <$mt:CategoryLabel$>
category.
</mt:Categories>

jdriddle
May 6, 2009 1:22 PM | Reply
"If invoked within Categories this tag will behave as if it was an alias to CategoryCount."
This functionality is broken as of 4.25. Use mt:CategoryCount when in a Category block.
Gautam Patel
September 17, 2009 4:55 AM | Reply
All counts take the following additional (undocumented) attributes:
Where sometext is whatever you want.
Thus, this could be:
and it will say "1 entry", "2 entries" (or whatever number of entries it finds) or "No entries"