How to access the sprite sheet object

I loaded a sprite sheet using:

this.load.spritesheet('tuna', 'assets/SPARK/ships_green.png', { frameWidth: 32, frameHeight: 33});
}

How can I access the object of this spritesheet to say, for example, see how many images are loaded? And if I wanted to use a for loop to iterate through the list of sprites and add them one by one. Is this possible?

Yes, see create() in load texture atlas. Same method for a spritesheet texture.