How many species of mites live on the human body?
The answer is 2-3 human mites. 

This page says that 2 mites specialize on humans: 
http://www.bbc.com/earth/story/20150508-these-mites-live-on-your-face
https://link.springer.com/article/10.1007/s10493-009-9272-0

Another page I found said there are 3 common human mites, adding scabies to the list.

This page talks about bites from a variety of dust and house mites: 
https://link.springer.com/article/10.1007/s10493-009-9272-0

My biology lecturer from university of Edinburgh told us that we have hundreds of species of detrital nematodes living on and inside us and that nematodes can be a few microns long. About 800 million humans have a 30cm large nematode inside them according to the WHO.