Probably about an inch or so. Depends on how many worms I have at the moment. Enough so that most of them can comfortably be buried in the oats if they want. I usually have a couple dozen on the surface.
I feed them potatoes and carrots mainly, alongside any left over salad or fruit items I have lying around on occasion. For instance, they just devoured a slice of mango lol. Every once in awhile I'll put something sweet or juicy in there. Just make sure you remove it after awhile to prevent any mold growth. In that past i've also put in roach chow for some protein. I tend to treat them like roaches, I'll give them any scraps I have basically. Surprisingly, they even will eat the collard greens I use for the dragons too. They aren't too picky.
-Brandon