I have previously answered a question in this post here on the early Han dynasty conscript system, which was nearly identical to the Qin conscript system during late Warring States and early Imperial Qin periods. Qin under the reign of King Zheng (who later became the first Emperor Shi Huangdi) would have more than a million people in reserve, in the sense of the number of male population b/w ages 17 and 54 who have received basic military training and are ready to be drafted if needed. The actual number people Qin mobilized for a given military engagement almost certainly never came close to a million. If we go by the records written in Shiji, the largest military force ever fielded by Qin was no more than six hundred thousand soldiers during its conquest campaign against the State of Chu. Even this figure includes both combat and support personals, and the number of deployed Qin fighting force during its Chu campaign was more likely to be around two hundred thousand men. This is nonetheless still a mighty military force for the time, but certainly far less than a "million men army." With the help of sweeping policy reforms instituted by Legalist statesmen such as Shang Yang and Li Si, Qin was known to be highly efficient in terms of enforcing general mobilization and training regime of its population, and effectively implementing a merit based award and punishment rule framework. Qin was also more successful than many of its peer competitors in rooting out internal feudal power structure, which helped centralizing the war mobilization power of the Qin state. That said, by the very end of the Warring States period, all major Chinese states have implemented similar general mobilization policies, and Qin army did not have a clear numerical advantage than its major peer competitors. For example, during the pivotal Battle of Changping (262 BC to 260 BC) both the State of Qin and the State of Zhao had mobilized armies of up to half million men on each side. Most of the mobilized men were conscripts, and both Qin and Zhao had to regularly rotate their army personnel in and out based on the needs of crop harvest back home.

For a more detailed description of how Qin conscript system worked, here are relevant passages I wrote from my earlier post linked above:

To get a picture of how slavery in early Imperial China (Qin and Han dynasties) worked, it is important to understand the role of the imperial authority in terms of meaning labor relations within realm. There are two broad premises to keep in mind here: First is that the imperial authority assumes the monopoly of the legitimate use of bonded labor within its territory, not only within the junxian / 郡縣 (imperial provinces), but also in the domains of its ennobled clans. Second is that for all practical purposes, freedom of movement and labor as we understand today simply did not exist throughout Imperial China. This is particularly the case in early Han dynasty, where by law all classes of people within the Empire, from common farmers to members of the literati or even the highest ranking nobles, are by default confined within their designated line-of-work and domicile. Unauthorized travel and change of profession are not only frowned upon, but considered to be outright criminal and seditious. There is also mandatory fixed term conscript “labor tax” required for all adult male civilians in good standing (shumin / 庶民), known as the yaoyi system, which I will discuss further in paragraphs below. On top of this, there are also various penal labor systems, including the rather odd “voluntary” self-enslavement system in place (more on this later). In short, in early Imperial China (or really much of the entire Imperial Chinese history), it is very difficult to draw a clear line and say this is where “free” labor ends and slavery begins.

The Qin and Han dynasties inherited two main forms of involuntary labor systems from the Warring States Period (403 BC - 221 BC): Yaoyi / 徭役 (“conscript service”) and nubi 奴婢 (“penal serfs”). Institutions of yaoyi and nubi in fact persisted throughout Imperial Chinese history (only to be formally abolished in the early 20th century). Furthermore, yaoyi service and literati bureaucracy altogether are considered the backbone of Chinese imperial power structure.

Yaoyi is a state managed conscript labor system emerged during the Spring and Autumn period (776 BC - 403 BC), and was adopted by virtually all major Chinese states by the mid to late Warring States Period. Prior to Ming Dynasty, Yaoyi has always been framed as a form of taxation, a duty all male shumin (庶民, “commoners”) in good standing are expected to pay in the form of their labor. If you are wondering what about those who are not “in good standing,” they are of course covered by penal labor systems which I will get to later. Now back to yaoyi, it is considered one of the three main forms of taxes collected by the state.

The yaoyi system during the early Han period was almost identical to the one used by its Qin predecessors, and was used to provide the manpower for the Imperial army as well as unskilled labor for various civilian and military projects (canals, roads, dams, fortifications etc). As its name suggests, yaoyi laborers are conscripts, it is involuntary in nature, but nonetheless they are paid and are usually provided with adequate housing and food. While the Book of Han claims that its preceding Qin dynasty operated a much harsher yaoyi system with mostly unpaid laborers and high casualty rates, current archaeological findings from the Qin period reveal a quite different picture. Textual records from Qin periods shows that their Yaoyi system was nearly identical to their Han counterpart. For example, one of the surviving《云梦睡虎地秦简》Qin bamboo slips from 217 BC contains the following lines stipulating laws regarding payment for drafted laborers:

“有罪以貲贖及有責於公 以其令日問之 其弗能入及賞 以令日居之 日居八錢 公食者 日居六錢” (My rough translation: “For those conscripted by the state for overdue debt or tax payments, all of their workpay must go towards their outstanding debt or tax balance, and they shall receive no bonouses. For regular conscripted laborers, the pay would be 8 qian per day if he commutes from his home everyday, and 6 qian per day if his lodging and meals are already provided by the state.”).

While the length and frequency of yaoyi services varied from time to time, generally all Han adult males outside of the elite literati and nobility classes are required to serve three categories yaoyi services: Zhengzu / 正卒, **Gengzu/更卒 and the infamous Shubian/戍边. Zhengzu (“main service”) is a single two year military service all male civilians are quired to serve at age fifteen. Han law allows delaying the draft for those households with elderly parents and the son to be drafted is the only adult child. The first year of zhengzu service begins right after the final fall harvest each year, where the county commissioner would assign new adult males of his jurisdiction into a local Wu /伍 (“platoon of fives”), and they will be drafted to the local Jun (prefecture) military camp for one year of basic military training, then one more year of service near the Capital as the reserve force. This is the system of which the Han empire maintains its regular standing army. All male commoners in good standing in Han are regimented into Wu’s throughout every aspect of their adult life. Members of the Wu must be non-relatives, rotating on an annual or biannual basis, with the most senior member assigned as the Wu leader. Under Han law, when when a single member of the Wu commits a crime, all five of them will be punished collectively (almost always in the form of penal labor, which I will discuss in the next post).Gengzu/更卒 are shorter local conscript services lasting from a few days up to a month. They are for all sorts of local civilian projects, such as road maintenance, digging canals, expanding irrigation systems, clearing lands, repairing a bridge and so on. Gengzu are issued throughout the year by the local county government on a need based basis.