As you all know, Dragon's Crown's 2D graphic is extremely high quality.

How to make this sort of assets? And how to make animate it?

Draw each frame's static 2d image? 10~30 images for 1 second animation?

If so, how input all those thousand textures into game's resources?

How many developers needed for this(not for scale of Dragon's Crown, but much much (1/10) small scale 2d game)?

It looks to me like they are simply high resolution sprites with lots of animation frames, though I don't think it is as exaggerated as 30 images per second. But it wouldn't be like a 4 frame walk cycle either most likely.

I don't see many indie games on this scale, unless they have a dedicated artist, or even more than one. Even a game that is much smaller will still need plenty of pixel art, and at that high of resolution pixel art takes lots of time.

As far as inputting those into the game's resources, I'd say it depends on the game engine. For example, Gamemaker Studio has a concept of texture pages, and the things that should be put together you put into these groups. It is similar to tile sheets, except that more than one actual texture can be in the same group, and they get loaded all at once into texture(video) RAM. When changing levels, you call a function to clear the textures and then load in the new ones as you draw things. If you make your own engine, you will probably want to do something similar, and then I'm sure any optimized 2d engine will do something like this, or allow you to do it yourself. It may seem like these textures are huge and take up space, but offline space is really not much of an issue anymore except for online only games, as even mobile games can be a few GB anymore. And you don't have to have all of that loaded into memory at any time, rather only the stuff you need to actually draw at the moment. In reality, this applies to everything, including objects, sounds, music, well....everything. It is part of the general optimization you would do to make a game run fast enough.

My guess is that this is the same graphics method used in Okami. I'm looking at the part of the video from 16:04-16:30. I think that they are making rough 3d models, then painting over the model animation frames for the characters. The background images are a mix of painted-over renders and freehand paintings mounted on simple polygon shapes for moving them around within the 3D world, especially rotational animations.

(And BTW the chest on that female character is disturbing. o_o )

For less "disturbing" (;) ) chara design but the same graphical style, I would recommend the other games from the same studio (Vanilla soft), there is the sublime Odin Sphere and the somptuous Murasama. (Yeah, I like superlatives and theses games)

I guess they are using the same technic.