Unless overridden by specific hardware, driver, or operating system settings, FMOD automatically downmixes its output to match that of your output device.
7.1.4 surround is the largest number of channels currently supported by FMOD Studio, and which can be handled by its spatializing effects. However, the FMOD Core API can handle larger numbers of channels.
Do you have a specific use-case in mind that requires more than twelve channels?
We already support Ambisonics through the Resonance Audio Soundfield effect. It allows you to load and play ambisonics files.