It's your project and you can call things whatever you like; still, the 8 free cores seem to me more like a "horde" than a "hoard".
Generally things hoarded are sequestered on a shelf against some imagined future need instead of put into service. At the same time, a "horde" usually numbers more than 8, except when treated ironically, after the manner of "three's a crowd". So I would suggest "horde", or "crowd". 8 is just a bit thin even for a crowd, unless it concerns a certain president's inauguration attendance.
Oops, changed to 'horde'! The distinction is mostly architectural, in that the 'horde' is composed of an undifferentiated mass of CPUs (currently 8, could probably be expanded up to 128 with a cache-based memory hierarchy, which would certainly feel more 'horde-like'). The application processors can be accessed individually, whereas the horde can only be accessed as a group.
Generally things hoarded are sequestered on a shelf against some imagined future need instead of put into service. At the same time, a "horde" usually numbers more than 8, except when treated ironically, after the manner of "three's a crowd". So I would suggest "horde", or "crowd". 8 is just a bit thin even for a crowd, unless it concerns a certain president's inauguration attendance.