I used 20 on a site. I got rejected from
http://sex-hot-sites.com/
The rejection e-mail was 20 recips is way too many recips.
I replied with "How may recips should I use?"
The reply email was " 16 should be the maximum."
Lately surfing other sites. I have found that 20 is the norm. I am assuming that the submissions are from "trusted submitters". It's like hey bro I'll let you slide..... .