I have 5 characters on my XBox. When you start a new save, the game creates a new save that starts at the beginning of the cart ride with a Nord named prisoner, save over that. Eventually, the Auto saves will start appearing, save over the file that has the numbers and not the AUTO labeled files. When you have a lot of saves like me, the save menu will look like this.
(Not actually a spoiler)
Spoiler AUTO
AUTO
AUTO
371 (Save over this one)
256
198
119
57
The Auto saves will move to the top of the list if you travel through 3 cells (The game usually autosaves when you walk in or out of a building/dungeon/alternate dimension/etc.), and your most current save will always be right under the third autosave. The saves under the current saves are your other characters. The number indicate how many times you have manually saved. (I'm up to 437...)