Hi adiboy, Yes you can, take a few minutes to read their TOS: https://www.google.com/adsense/support/bin/answer.py?hl=en&answer=48182 Good luck!
"Site" is a meaningless metric unless the number of pages and the extent of content is also mentioned. 20 one-page sites is a LOT less than what's on any particular one of my sites.
I am using adsense code on more than 20 websites and blogs without any problem. Very well said Adpubster Tehmina