WP multisite, subdomains, sitemaps and seo.

Hello, i have some questions regarding wordpress subdomains and sitemap and seo.
I have a website, site.com with installed a parent theme and a child theme.
I made a multisite wordpress, with main site site.com.
For my needs i had to create one little website for each region ( total 20), this because all the subdomains are regional points for the business.
So i made 20 subdomains: region1.site.com , region2.site.com and so on.
All those subdomains are taking the theme from the same child theme of the main site: site.com.
Each subdomain will have only 2 pages (home page and contacts)
After this i need to create the sitemaps.

My questions are:
– should i create a special sitemap only for site.com ?
– should i create 21 sitemaps, 20 for the subdomains and 2 for the main site?
– there is a way to explain to google that all the subdomains are just the regions of the nations? and so if google see some duplicate contents he would know why?
– Do this subdomains thing affect my seo on the main site?
– can i create conflicts using the same child theme for all the subdomains? should i create one child theme for each subdomain?

thank you very much