You cannot use more than 12 inserts for the arranger engine. One insert for each of the 8 style parts + one insert for each of the 4 left/right hand parts. Multi pads, with the builtin firmware, incredibly can NOT be effected.

The remaining 16 inserts are usable on the second midi port (the one for midi file playback, which generally speaking anyway do NOT use those 16 Yamaha specific inserts that can be exploited through convoluted sysexes only).

12 inserts are still a good number (probably more than any HW competition) and more than enough (most builtin styles do not use all the 12 possible inserts!). But nonetheless nowhere close to the 28 listed on the spec.