I've just been introduced to the notion of loyalty portraits, a portrait of a British house's servant staff made for their employers, in this documentary about service. Some time back, I see a large number of them went on display
The National Portrait Gallery will feature “Below Stairs- 400 Years of Servants’ Portraits
This exhibition presents around 100 portraits spanning from the 17th century to the 20th century and includes not only domestic servants but also institutional staff, for example the porter from the Royal Academy, the Arts Club cook and the British Museum’s housekeeper.
Sounds like there's quite a lot of them in existence. Is there anywhere online I can learn more about them or see some of them? Googling has pointed me mainly to the documentary I linked. I'd love to learn more about this time and this practice.
Portraits of servants, court entertainers, and even slaves are not uncommon in early-modern Europe.
Looking for printed books may be easier than searching online. The best resource you are going to find is the catalogue of the Below Stairs exhibition. You may also be interested in the catalogue Perfect Likeness: European and American Portrait Miniatures from the Cincinnati Art Museum and the edited volume Slave Portraiture in the Atlantic World.