Forerunner fleet size 1

"At the height of their power, billions of Forerunner starships plied goods and ferried passengers between stars, and enforced their vision of peace on the galaxy."
(Halo Warfleet; pg.8)

"In support of 05-032's original 1000 core vessels is a fleet numbering 4,802,019; though only 1.8 percent are warships - and only 2.4 percent of that number are capital ships - I am outnumbered [436.6:1]. I expect my losses will be near total, but overwhelming force has its own peculiar drawbacks."
(Halo 3; Terminal 6)