When Madred tells Picard "the Enterprise is burning in space", he probably uses a figure of speech rather than the literal meaning of "burning".
Jellico orders Worf to prepare 500 antimatter mines to attack the Cardassian ships in the nebula. Later, Riker flies the shuttle with Geordi to place the mines, and there are only about ten of them. Indeed, based on the size of these ten, 500 would be much larger than the entire shuttle.