I am trying to convert my Dolby ATMOS 7.1.4 .adm files to MP4 files. The conversion works but I am only getting 6 channels.

In many players, the MP4 will appear to be a 5.1 file. This is meant for backwards compatibility. On more advanced (i.e. Atmos-capable) players, the Atmos metadata will be read and the file will play true DD+JOC to any number of channels. Best way to confirm this is with the Dolby Reference Player, but even an Xbox or other consumer device can be used, too.