You can add any number of artwork items in a theme. The process is simple:
1- Add the artwork in [emulator].cfg in AM/emulators. For example to add flyers & fanartin NES, open NES.cfg (or whatever you named it) and add
artwork flyer Path\to\flyer folder
artwork fanart Path\to\fanart folder
2- Add a line in layout.cfg about the new artworks:
local flyer = fe.add_artwork ("flyer", 500,500,800,600);
local fanart = fe.add_artwork ("fanart", 500,500,800,600);
You can use following for performance boost
flyer.preserve_aspect_ratio = true; #retain aspect ratio
flyer.mipmap = true; #great for items which are scaled down
flyer.trigger = Transition.EndNavigation; #this will update artwork item when you stop scrolling i.e. performance
(use fanart instead of flyer in above mentioned code or anyother art item. You can add any number of artwork items afaik there is no hard limit)