Hello

I am wondering how to add custom dragon breeds, I have the whole file setup/texture but I don't know where to put it.

Also can you put multiple dragons into one pack?

The way you would structure it is as follows:
The json that describes the breeds attributes and stats will be part of a datapack.
The textures, sounds, etc. would be part of a resourcepack. In order for this whole system to work, you need BOTH.

The difference between the two would be the datapack carries the data folder, and the datapack needs to be placed in your worlds datapacks folder.

Resourcepacks carry assets and should be placed in your game's resourcepack folder.

iirc both of these folders may be accessed in game through a button.

The structure of how these packs are built is with the example here.

Hope this helps!