Here is a long list of human colonies. Pre-war populations included. Post-war colonies are now included. Every region of human-controlled space has a certain number of heavily populated worlds. The Core Systems have a "Core Twelve", the Inner-Rim has a "Core Fifty", the Mid-Rim have a "Core Eighty", and the Outer-Rim have a "Core Ninety", each heavily populated world's population's exceeded one hundred million citizens. Some of these names came from Halo Fanon.
Update: I will be separated each region of human-controlled space into different chapters soon.
Under the administration of the United Republic, humanity has surveyed and colonized a vast number of worlds across the Orion Arm of the Milky Way galaxy over 200 years of superluminal travel via slipstream space, following nearly three centuries of expansion across the Sol system. The planets under the jurisdiction of the URE, together with humanity's homeworld, are collectively referred to as "Earth and Her Colonies". As of 2525, a total of 1,248 colonies had been established, many of them firmly under the influence of the UEG with some seeking to secede from humanity's empire. The worlds are highly diverse in terms of culture; some worlds have their foundations in the cultural heritage of a single Earth nation, while others have more varied population bases. After the Human-Covenant War, some worlds became even more heterogeneous with the emigration of former Covenant client species to the human sphere.
At its peak, human-controlled space encompassed hundreds of star systems and planets, moons and planetoids settled by colonists, some terraformed to greater degrees than others depending on the local conditions and presence of indigenous life. The colonies ranged from heavily-populated and economically, politically and strategically significant worlds to struggling pioneer outposts which could barely sustain 10,000 colonists. The initial colonies were the Core Systems, a region of 2.8 to 9.1 light-year three-dimensional radius from Earth. The Inner-Rim Colonies encompassed a region of space in at least a 10.5 to a 28 light-year three-dimensional radius from Earth. Beyond that were the Mid-Rim and Outer-Rim Colonies. Other colonies were established even further - Coral, at 42 light-years; and Madrigal, which lay in the 23 Librae system, 83.7 light-years from Earth. Harvest was considered the furthest Outer-Rim Colony in 2525, at six weeks of Slipspace travel away from Madrigal and two months from Reach.
While the United Republic is technically a federation of equal, independently-governed worlds, this is not true in practice; the interests of Earth, Core Systems, Inner-Rim Colonies, and some Mid-Rimmers, weigh more heavily in the government's decision-making than those of the Outer-Rim Colonies. This has led to tensions and even civil war between the UNSC and the independence movements based out of most Mid-Rim and Outer-Rim worlds, which many of the Core Systems and Inner-Rim Colonies relied on for their foodstuffs and resources.
Various organizations exist to provide military defense for the colonies. The Colonial Military Administration was created to act as a unified security force for the colonies but was eventually sidelined due to widespread corruption and defections. The CMA's role was taken over by the United Nations Space Command, which many colonists resent due to its perceived draconian influence, though the UNSC is attempting to change that part. At least some colonies maintain their own territorial defense forces, including Sedra's Colonial Guard. Anti-UNSC sentiments are not uncommon among these organizations, some of which—such as the Venezian militia—are openly secessionist. The UNSC also operated its own Colonial Militia program to train colonial citizens to deal with emergencies as well as local rebellion.
The URE has always been considered one of the largest nations in the Milky Way for the large amount of colony worlds under its control.
AN: This comes straight from Halopedia, just modified a little.