I suppose, that it depends on disk partition map not OS.
On Powermac you have probably APM? ( under MorphOS you see it like "MAC" )
In any case, APM and Amiga RDB supports really big number of partitions.
APMRDBHere is probably no limitation from MorphOS, but I am no 100% sure. In any case, I am using under MorphOS many partitions, I write exact number where I will be at home.
AmigaOS3: Amiga 1200
AmigaOS4: Micro A1-C, AmigaOne XE, Pegasos II, Sam440ep, Sam440ep-flex, Sam460LE, AmigaOneX1000
MorphOS: Efika 5200b, Pegasos I, Sam460LE, Pegasos II, Powerbook G4, Mac Mini, iMac G5, Powermac G5 Quad