Hmm, in the official mission help file it says it will check all fleets when fleet number is not specified.
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CONDITION: if_fleet_count (tests an indexed fleet's membership count against a condition)
ATTRIBUTE: name
VALID: text
ATTRIBUTE: comparator
VALID: =, !=, <, >, <=, >=, EQUALS, NOT, GREATER, LESS, GREATER_EQUAL, LESS_EQUAL
ATTRIBUTE: value
VALID: signed floating point value
ATTRIBUTE: fleetnumber
VALID: 0-99
if you omit the fleetnumber, this command will count every enemy in the game
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I only made an editor, i didnt have time or patience to check every single mission statement to actually work as intended - even though, i found some problems and shared what i found, even found a grave problem with game's > and < comparison, outlined here
http://www.artemis.eochu.com/?page_id=28#/20120514/severe-bug-in-artemiss-mission-scripti-parser-1582416/ One single wild idea - try to manualy (through text editor) edit the line to:
<if_fleet_count comparator="EQUALS" value="0" />
That removes the .0 from the 0. If it works then - this might mean another bug in the mission scripting, that 0 != 0.0 (integer zero != real zero)