How many is the #of ballet dancer in a group?

It all depends on the size of the budget, which is dependent on the amount of donations a company can bring in, as ticket sales cannot cover the whole budget. Thus, you can have a company of 15 dancers in Grand Rapids, Michigan..

http://www.grballet.com/gr_ballet/AboutU...

and a company of 99 dancers in New York City..

http://www.nycballet.com/company/dancer....

(click on "Dancers by Rank" to see the list).

Saying "as many as the dance needs" puts it backwards, as a company will tailor their repertoire to fit their size. They can also enlist the assistance of student dancers to expand their ranks for shows like "Nutcracker." But those student dancers are not members of the company.

How many is the #of ballet dancer in a group?sunshine

Eleven. Just like football.

How many is the #of ballet dancer in a group?star theater opera theater

A group? You mean... like a troupe? Or a company? It depends.
like a company? or a troupe? as many as the dance needs... hope this helps...
It depends I think.
It really depends. There are companies as small as thirteen and as big as more than 100. Generally the more prestigious/famous the group the larger it is though I think .