Covenant casualties had to have been in the hundreds of thousands during the month long battle for Reach. The covenant corvettes at least have a crew and ground numbering in the region of several hundred to at least a thousand strong. In the early stages of the campaign there are at least three of these ships attacking Visegrad and the Ice shelf. A Covenant Assault Carrier is estimated to be able field as many as 10,000 or more warriors including the crew.
A Super Carrier at 27 kilometers as to opposed to an assault's 9 kilometers is likely able to hold as many as 50,000 soldiers and various large armoured vehicles i.e. Scarabs, Wraiths, locust walkers etc. This would explain why the covenant had so many soldiers to start with and at such an early stage in the invasion.
It's likely that their were two main large covenant fleet deployments. the First one numbering 300 strong is the fleet to arrive shortly after the destruction of a Long Night Solace. this is the fleet that Keyes and the first reinforcements face in the book the fall of reach. The Second deployment arrives shortly after the destruction of the Super Mac generators. Around the same time as the level Pillar of Autumn begins, this fleet numbers 400 strong and thus has 700 Covenant ships at Reach.
This total was confirmed by both bungie and 343i and appears in the new edited version of the Fall of Reach. (sorry for those who all ready know all this, just making sure people understand).
Anyway my point is that with such a large and never before seen such large numbers of Covenant warships. Since the smallest vessel has at least a few hundred to a thousand troops and the largest as many as 50,000, it is quite possibly that the covenant as whole numbered in the millions at Reach.
As other people have stated, Humanity normally won the battles on the ground and tended to have higher kill ratios than their alien opposites in terms of ground combat. Since the Covenant would have needed troops numbering in the tens or hundreds of thousands to subdue such a well defended and important military target as Reach. then the casualty list must have been incredible, especially as at least two hundred covenant warships were destroyed during the battle out of the over all 700.
So, yes Reach was the hardest and bloodiest fight that both sides had experienced since the early years and battles of the war. Only the battles of Earth surpass reach in terms of scale and loss.
Q. How many civilians made it off reach? does anyone know a figure or a source saying how many civilians survived the fall of reach and by survive I mean flee off world?