There are three gardens total. You start out with the Station Square Garden and can obtain the Mystic Ruin and Egg Carrier Chao Gardens (look below for more info). Each garden can hold 8 Chao, and when first unlocked, 2 Chao can be found walking around.

This garden is obtained by going up the left elevator in the Station Square hotel. It is a plain garden, with most of the ground being marble. It also has a fountain and a little grassy area where three trees grow. This is where the Black Market and Chao Races are.

You get this garden by going to the Egg Carrier and finding a door that needs a password (which is EGGMAN). You'll notice a set of buttons on the ground, those are what you use to write the password. This garden doesn't look at all like the Egg Carrier, instead it looks like a beautiful beach. It has several boulders around one side, and a missile sticking out of the ground on the other side.

This garden can be found in the cave to the left of Tails' workshop. Once there, you should see a cart, jump in and it will take you to the garden! This garden, in my opinion, is the best. It just has an awesome feel to it. It has a lot of platforms for Chao to fly off of. It is also covered in lush green grass, and has a small waterfall. A cool little fact is: When it is nighttime at the Mystic Ruins, its also nighttime in the garden!