Okay so after scouting out some numbers something just didn't compute to me. 343i issued a statement regarding the population of Earth pre-invasion and post invasion.
They said pre-invasion: 10 billion
During invasion: 200 million
Post-invasion: Several billion
Now we know that according Osman the Battle of Earth resulted in a "few billion" casualties. So lets say around 3. That still means that the UNSC was able to evacuate roughly 7 billion people.
Now we know even as of September 4, 2552, the Fall of Reach was known to all but the top brass, so its doubtful evacuations had begun until then. We know the First Battle of Earth started in October and ended in November. Evacs seemed to be fairly complete as of November so lets say November.
The current warship to merchant vessel in the world currently is roughly 1:17. We know the UNSC had roughly 2000 warships at the start of the war. So based on that statistic there'd likely be around 33,000 merchant vessels. Additionally there'd likely be about 250 cruise liners. If each vessel held 10,000 people (which is totally possible if they are all roughly the size of a cruise ship today. 33,250 vessels could transport 330,250,000 people in one trip. Well we know Reach is 10.5 lightyears from Earth and is its closest colony. So lets say the places they were about 20 lys away. At a rate of 2.65 lys/day that means roughly a trip every week and there'd still be only 2.642 billion evacuated by the time November rolled around. If each vessel held 20,000 people that'd jump to 5.284 billion. If we say there were 43,250 vessels instead that'd put it at around 6,920,000,000 billion evacuated which is right in the ballpark of the survival rate on Earth.
So the combined Earth Fleet is likely (numbers wise) to be approaching the size of the Quarian Migrant Fleet in Mass Effect. Wild.