We know that there are at least 800 colonies before the Human-Covenant war, but how many are left? The Covenant supposedly only bombard strategic planets to an uninhabitable state (or 'glass') or the ones that show great resistance, and the number of glassed planets is thought to be 76. However, 'virtually every outer planet' was glassed by 2535, 18 years before the war ended. So if the Covenant had the outer region of the Human empire for 18 years, then all outer colonies were presumably taken.
Many colonies like Arcadia, Harvest and Reach were featured in the games, possibly signaling that they showed much more importance. But all of these colonies were eventually somewhat glassed, possibly raising the ratio of awful defeat when put next to victorious defense. If there is a lot of these remaining colonies left, then that might leave lots of room available for future spin-offs and trilogies. What about revisiting glassed colonies? Those colonies that were so quickly taken by the Covenant would make a great future environment in new games. What about the Forerunner bird in the Halo: Xbox One trailer? That desert looked a lot like a glassed environment that had some dust covered on top of it. So maybe fighting back against already taken colonies will be an objective in Halo: Xbox One, which was rumored to release in 2015 instead of 2014. Many of the colonies fates were left undecided and forgotten by the authors, but could this be on purpose, or did the authors just forget to put a pin in it? If the outer colonies were a major part in upcoming Halo games, then (as I explained above) the upcoming Halo games would feature a lot of creepy glassed ghost towns.
What about the ecological effect on these planets? Some of the animals and plants that perished on the captured/glassed planets could have clones or some sort of specimens that could make terraforming possible. Ironically in Halo 3: ODST, one of the first places on Earth that was attacked was a major wildlife exhibit, possibly containing the last specimens for extinct species. Since Earth was the last stand for a measurable defense against the Covenant, most of the important stuff like specimens and rare animals had to have been kept there and because Earth was almost completely overrun by the Covenant during the last weeks of the war, these vital information hotspots had to have been damaged to some extent.
Could the UNSC have taken glassed colonies (other then Harvest) back? Right after a planet is claimed by the Covenant, some of the fleet probably would leave immediately, but how hard would it be to take back a forgotten colony? Would the Covenant make massive permanent outposts or refitting stations on these barren wastelands? Or would it just be a pit-stop refueling station for the occasional Class-CCS battlecruiser? The first theory would be a reasonable answer for a colony in the Inner colony range, while the latter theory would be a good answer for far away outposts. The first theory including a large refit station would be well defended, so taking back close captured colonies might be difficult. In Halo: First Strike however, a scenario much like the one above was successfully executed, leading to the possibility that large Covenant stations have been destroyed by Special Operations SPARTANs. If this has happened more then once in a blue moon, then some captured colonies could have been reclaimed.
But what about the Inner Colonies? There are only 5 known systems in the Inner colonies; The Epsilon Eridani system which has Reach, The Solar system which holds Earth, the Sigma Octanus system which has Sigma Octanus IV, the Ectanus 45 system which holds Chi Rio and the unknown system that holds Skopje. The Inner colonies page brings up a point on this, seeing that almost every Inner colony that was attacked was attacked during the last years of the war; thus a lot of Inner colonies presumably survived. Earth's contact with other colonies is mentioned in the book Halo: Glasslands, where many colonies independent governments don't want to cooperate with the United Earth Government, after most of their military protection was taken to ensure Earths survival. But these independent and rebellious Human colonies will still be marked as 'surviving' colonies. Back to what I was saying earlier, many of the Inner colonies presumably survived because hardly any were attacked before 2550. Seeing as how the war ended in the earliest days of 2553, the Covenant only fought hard in the 'Innies' territory for roughly 3 years. Doctor Halsey states that not only do colonies fall in weeks, they fall in days. Planets can be completely rendered glassed and uninhabitable in only a few hours, depending on the fleet size. Given how other then Reach and Earth, there is no major military stronghold capable of repelling invaders for over a month, a colony found by the Covenant is a glassed colony by the Covenant.
So a big question in the Halo universe that everyone would want to know about, what is the current condition of the colony Harvest? Harvest was one of the most visited battlegrounds in the entire war and thousands of troops had come only to perish in the unforgiving world that is Harvest. But is it still habitable? The Covenant left Harvest leaving it somewhat glassed, until they returned to find the Forerunner artifact hidden there. Over the course of Harvest's skirmishes, it was left only to be a 'nuclear winter' planet, an effect caused by the blocking of sunlight after derbies are forced into the atmosphere. However, some regions of Harvest appear to be habitable. So Harvest very well could be safe, and we may come back to it. Some fans of Halo have made minor pushes towards 'Halo Wars 2', so if that ever comes out, it may follow the later years of the Harvest Campaign.
The remaining colonies (presumably) could total up to a maximum of 700, even though that is really unlikely. Plasma weapons need to be recharged too, so glassing probably costs a lot of resources even to the Covenant, making the 'captured but not glassed' factor enhanced greatly. Because plasma based weapons are unimaginably expensive in the real world, they must be somewhat expensive to the Covenant. So the number of surviving planets (However highly unlikely) could reach 700, but is most likely to be in the 200-500 range. Even that is an overkill, so the true number might never be known.
Even though we may never know the true number of surviving colonies, we will know that half of the Human population survived, making a strong variable on this topic. Please leave your comment, opinion and what number of surviving worlds you think are left.