Forerunner fleet size 3

Reluctantly, as if unveiling a new nakedness, he magnified and shifted the images for all to see. Thousands of ships were arrayed in clusters around the wide-vaulting star roads. These old ships were massive enough—most in the range of one or two kilometers—yet sleek in their obvious power, and, to my eye at least, thoroughly aggressive, deadly looking. Yet indeed of an oddly familiar pedigree, as if even what the most ancient Forerunner voyagers had wrought was still recognizable to their descendants millions of years later.
“It must have been horrendously expensive to bring them here,” Chant-to-Green said.
“Almost certainly—if just moving our tiny ship nearly bankrupts the ecumene!” Keeper said. “But why? What were they doing here?”
“Cheaper to abandon them once their work was finished,” Clearance said, breaking from his trance.
“But what was their work?” Keeper asked, clearly frustrated, conflicted.
“Any dozen of them could have surveyed an entire system,” I said. “But we see hundreds of thousands.”
“A tremendous fleet—and clearly a battle fleet,” Clearance said. “Sent here to kill on a massive scale.”
Indeed, a fleet of this size could have targeted myriads of stars and planets—and how many of those worlds had once been inhabited by Precursors?
(Halo:Silentium; Ch.8)