The longest chain of parasitism that I’ve been able to find:

https://www.biodiversitylibrary.org/part/28549

Interestingly, in order to support many levels of parasitoids, each level has to be extremely efficient, managing to use nearly all of the body mass of its one host.

See Interactions to the fifth trophic level: secondary and tertiary parasitoid wasps show extraordinary efficiency in utilizing host resources