One domain, 21 installs in subdirectories, How many sitemaps.

I’m not using multisite yet but…

I have a directory site: “mydirectory.com” that consists of 21 WP installs in sub directories (…/home, /aerospace, /automotive. /mechanical, etc.). The directories are all interlinked and the site is working fine for me.

I am confused about the sitemap(s). Should I put one big all-inclusive sitemap in the home directory or put a sitemap on each of the installs.

Any help or insight is appreciated.