Lothric knights drop them a lot. Other than that, they're as limited as the number of players who are willing to summon you to help for boss fights. So, essentially unlimited. One of my characters has over 150 Embers, and I'm sure that's nothing compared to what some players have. If you think you can't co-op bosses because you're not "gud" enough, forget all that. I'm nothing special at the game, I really suck at PvP, but I do well at co-op. There's been fights where I'm clearly better than the host and I've got to draw aggro from the boss just to keep them alive. Then there's fights where the host is clearly better than me but summoned me to do me a solid, I guess. Point is, pick your favorite boss, look up the optimal level and equipment upgrades for other people to summon you, and have at it.