The Compact has 1 array with 6 small speakers / transducers, and 2 extensions.
There are no speakers in the extensions.
It can be used with the speaker array installed in the base or with one or both extensions. Historically Bose has not recommended use with only one extension, but it will cause no harm to do so. Dispersion / coverage might be negatively affected with only one extension in place. Let your ears be your guide.