Sending several PhD applications to professors in the same university?
Your situation isn't that uncommon, actually. You have many options in front of you, and more than one appeal to you. There's no shame in that...

To answer your questions:


  How bad would it be seen if I sent applications to several professors?. 


I can't see why this would considered bad. If anything you are excited about applying to projects. If you have a good portfolio/application, and if you don't look too desperate there shouldn't be much of a problem. Just make sure that your applications are relevant and fine-tuned for each position. Nobody likes boiler-plate applications spammed all over the place..


  Will the professors even find out that I sent applications to their colleagues?. 


They may or may not, it depends on the requirement process (whether it's electronic/web-based etc) used by the institution, as well as the ethical considerations by these professors in question. If you are but a name to these people, what makes you think that they'll even recognize that you have applied to more than one position?


  How many are too many?


See the part above about boiler-plate applications, and spam. You don't want the person reading your application to get the feeling that you have applied to every possible position. If you can actually muster up a solid application to 15 positions, I say kudos and good luck! :) 

But seriously, you don't want to come across as "I'll do anything, just pick me!" Take your time, think about your interests, strengths etc.. And submit one or a few strong applications to positions that are most relevant to you.

If you don't get one of your favorites, but your profile fits some other open position they'll likely call you in for an interview anyways. Especially if it's the same group.