The exact prevalence of residual Ming loyalism is hard to gauge, not least because while anti-Qing sentiment could be quite common, conflating this with pro-Ming sentiment is usually unjustified. Take, for example, peasant uprisings against gentry landowners, whom the Qing supported. While these over time took on more of an anti-Qing character because of the Qing defence of the landlords (culminating in the Taiping and Nian revolts in the 1850s), the rebels were by no means pro-Ming either, as the Ming had in fact been brought down by peasant revolts sparked in part by the increasing consolidation of landholding.

Questions may similarly be asked about the Revolt of the Three Feudatories that lasted from 1673 to 1683, in which the three 'feudatory states', encompassing Guangdong and Guangxi, Fujian, and Yunnan and Guizhou, respectively, rose up against the Qing, proclaiming the cause of Ming restoration, when the Kangxi emperor tried to get their leaders to retire and thereby pass their domains into direct Qing administration, rather than retain them as hereditary possessions. Except... that's an oversimplification. The three feudatory princes, Wu Sangui, Geng Jingzhong and Shang Kexi, had gained their posts by virtue of having at a sooner or later juncture defected to the Qing, and indeed Wu had led the campaign that led to the capture and execution of the last Ming pretender, the self-styled 'Yongli Emperor', in Burma in 1662. While Wu nominally proclaimed that he was fighting his war for the restoration of the Ming, by 1678 he was declaring himself the 'Zhaowu Emperor' at the head of a new 'Zhou Dynasty'. Wu's anti-Qing uprising hardly represented a resurgent Ming loyalism, and the behaviour of the other two princes is hardly indicative of this either. Geng in Fujian surrendered to the Qing after three years of fighting and ordered his troops against Wu Sangui, while Shang in Guangdong in fact refused to join the revolt, and it was his son, Shang Zhixin, who brought the Guangdong feudatory into the revolt in 1676. Wu's previous defection to the Qing in 1644 had in any case badly damaged his credibility, and there does not appear to have been much in terms of popular support for him. Instead, it seems that it was personal loyalties to these individual leaders, rather than broader pro-Ming sentiments, that supplied manpower for the uprising.

Residual Ming loyalism appears to have been more an elite fad than a popular phenomenon. Some dissident scholars like Lü Liuliang defied the queue edict by becoming Buddhist monks and thus permitted to have fully shaved heads, and authoring subtly anti-Qing manuscripts that would be circulated among literary circles, but not broadly published. I'm not aware of any studies trying to quantify such residual loyalism, but that's likely due to my own unfamiliarity with the earlier Qing period – nonetheless, there are clear examples of Ming loyalist rhetoric among a small number of scholars even into the Yongzheng reign. Zeng Jing, a minor Hunanese scholar heavily influenced by Lü Liuliang's work, tried unsuccessfully to incite General Yue Zhongqi in Shaanxi to revolt against the Manchus in 1728, and thanks to the investigation into the incident inadvertently revealed the existence of Lü's seditious texts and their circulation. However, undercurrents of anti-Qing sentiment were also revealed through the response to the incident: the Yongzheng Emperor, through corresponding with Zeng, convinced him to repent, and so not only released the scholar but also gave him an official post (unusually) in his home province and a sizeable stipend, while also publishing their correspondence and requiring that it be read out in public on regular occasions, as a sign of the emperor's beneficence and sense of justice. In turn, however, a number of attempts were made by individuals to emulate Zeng's original act of treason by trying to incite other seemingly anti-Qing officials to rebel. Whether this can be linked with pro-Ming sentiment is again problematic, as realistically, by the Kangxi Emperor's death the vast majority of China's population would never have lived under Ming rule – the last Ming pretender was killed in Burma in 1662, Koxinga's attempt to capture Nanjing had been repulsed in 1656, and the Qing had taken Beijing in 1644. Appeals to Ming loyalism seem to have been part of a broader attempt at mobilising anti-Qing sentiment, rather than being seen as a realistic goal in itself.

My own inclination is to only view those who actually were of age during the conquest as sincere loyalists – people like Lü Liuliang or the historian Zhang Dai, who could actually remember the Ming world of old and who were in some ways unable to grasp the transition that had occurred. Those such as Zeng Jing or even Wu Sangui who used pro-Ming sentiments as a rallying cry against the Qing did so mainly out of rhetorical convention rather than literal, sincere conviction. I'd even argue that to some extent, the Kingdom of Tungning on Taiwan was not truly Ming loyalist either after a while, partly because of how unrealistic the end goal was from a practical standpoint, but also for the simple fact that the Zhengs on Taiwan did not attempt to produce a new Ming claimant or even stake their own.

Source materials on residual Ming loyalism are thus mainly constrained to the writings of those loyal elites, such as the largely destroyed memoirs of Lü Liuliang, or the largely intact memoirs of Zhang Dai. While I'm not sure how accessible these texts would be – especially not in translation – a couple of biography/microhistories do exist: Jonathan Spence's Treason by the Book is a great dive into the Zeng Jing conspiracy and Lü Liuliang controversy, and his Return to Dragon Mountain is an extremely compelling biography of Zhang Dai.