I am 16... I sing in the choir at my high school, and in the A Cappella group. I also sing in a men's chorus outside of school. So, I sing in front of thousands of people, whether I'm in a group or not. If anyone is interested, I will record myself and upload it to youtube if you want to hear me.
I could sing in front of over a hundred people, but they probably wouldn't like it
Depends on the length of the song, and whether I'm accompanied by an orchestra or a back-up choir or nothing at all. Also depends on the song. "Somewhere Over the Rainbow" might turn out a little awkward, but I bet I could belt out a rousing round of "Luv 4 dem Gangstaz".
Strangely enough, I would be more embarrassed to sing in front of a small group of people instead of a big group. Hm. I guess in front a small group you can see people's reactions immediately, whereas a large group would remain relatively hidden. In theater, I hardly notice the audience, because to me they are a sea of blank faces. I only notice them whenever they clap or laugh, though mostly the latter would happen if I were to sing