Hello,
you are right, the number of molecular orbitals is the same as the number of spherical atomic basis functions, while the number of basis functions per molecular orbital is the number of cartesian basis functions. The latter one is transformed to 6d/10f only.
I am not sure why the number of orbitals must be equal to the number of basis functions in AOMix-FO, but perhaps there is a possibility to get around this restriction...
Using cartesian basis sets should work, please contact the Turbomole support to get help in case it does not.
Regards,
Uwe