http://www.slate.com/articles/news_and_politics/politics/2015/05/the_gop_primary_is_crowded_why_are_so_many_republicans_with_no_chance_running.html

No one will blame you if you can’t keep track of the Republican presidential field. It’s huge. If you count declared candidates, prospectives, and announced aspirants, you have 18 people from across the Republican ideological spectrum: Sen. Ted Cruz, Sen. Rand Paul, Sen. Marco Rubio, Sen. Lindsey Graham, Rick Santorum, Gov. Chris Christie, Gov. Bobby Jindal, Gov. John Kasich, Gov. Rick Snyder, Gov. Scott Walker, Jeb Bush, Jim Gilmore, Mike Huckabee, George Pataki, Rick Perry, Ben Carson, Donald Trump, and former Hewlett-Packard CEO Carly Fiorina. The field is so large that news networks have put limits on who can join the debates. Fox News, for example, will invite only candidates who placed in the top 10 of an average of national polls. Likewise, CNN will hold two debates: one for top-tier candidates, and one for the bottom tier. (One possible effect of this? Underdog candidates will pull every stunt they can to get onstage.)