Depends on the original stereo option and, I believe, year (though I may be wrong about the year. Mine is a 2010 with the base radio which meant I had two 6X9s, one in each door, and two 6.5 inch rounds in the back deck. In 2010, I thought there were dash speakers too for the upgraded stereo. I changed the two 6X9s to Rockford Fosgate triaxials and I changed the 6.5 rear deck speakers to Polk Audio. The doors were a breeze to swap out though you do need to dremel tool the plastic housing doohickey that the speaker attaches too that then attaches to the door. Not too bad. The rears were a little more difficult to remove and replace and the optional Challenger specific mounting plate isn't exactly correct but it's correct enough with a little finesse and "english". Made a HUGE difference in sound. You will actually laugh when you see the contruction and magnet size of the original equipment speakers. I kept them just in case but you could literally throw them away and not think you were throwing out something valuable.
There are a number of audio options on Challengers, so it really depends on what you buy.
6-speaker Boston Acoustics
7-speaker Boston Acoustics w/ sub
Harman Kardon amp
With these cars it's not exactly like old cars where the speakers are driven by the head-unit and you can swap out the head-unit and speakers and install amps in-line. Head into the Audio section of the forum and look around, get some ideas, and ask your questions there if you can't find the answers.