In the Elden Ring DLC, there are approximately four story bosses. However, this can change depending on different gameplays and hidden challenges that might reveal additional bosses.
Well, as of July 2023, the DLC for Elden Ring hadn't been fully released yet. But generally, it's likely to expand on the lore of the Lands Between. It might further explore the origin of the demigods, their relationships, and the secrets of the Elden Ring itself. Maybe we'll get more backstory on characters like Malenia or Radahn.
We could see the expansion of the existing storylines. For example, the fate of some of the bosses or characters that were left a bit open - ended in the main game. There might also be new areas with unique stories related to them. Like, perhaps an ancient civilization that was only hinted at in the main game will be fully explored in the DLC.
Personally, I think it's better to play after the main story. You get to fully immerse in the main plot first and then expand your experience with the DLC.
Sure. Usually, game developers make DLCs somewhat independent of the main story's completion. But it might have some references or benefits if you've finished the story.
The DLC may expand on the main game's story. It could explore more about the demigods' pasts and their fates. Maybe new areas will hold the keys to understanding the Elden Ring better.
It depends on your preference. If you want to fully experience the main story without any additional content interfering, it might be better to play the main story first. Then, the DLC can be like an extended adventure after you've completed the main journey.