There are 7 main zones, so a total of must be collected. Finding them all is the most time-consuming part of 100% completion. A completed save file will have every Chao collected, meaning all Special Keys and Chaos Emeralds are also obtained.
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If you are playing on an emulator and looking to skip the grind, you can find complete .sav files on community sites: There are 7 main zones, so a total of must be collected