Fixing cracks is not a big deal. It took me a while to realize that polymer clay pieces can be rebaked an unlimited number of times, with repairs, new pieces added, etc. Your pieces appear to be tiny, though, so that will present a challenge.
If you have some of the exact same clay, put a small amount of translucent liquid clay into the crack. Let it set for a few minutes, then fill the hole with the matching clay. Smooth the surface carefully and bake the pieces again. You may have to gently wet sand the surface with fine grit sandpaper to get it to be smooth.
I don't think your pieces cracked because you had too many in the oven at once. I suspect it had more to do with the age/condition of the clay, the oven temperature, whether you mixed anything into the clay (translucent liquid sculpey, mineral oil, or clay softener) or some other factor. Check out Glass Attic to see if you can find anything about cracked clay. www.glassattic.com. There is an enormous amount of information there!