The 1.5GB limit per instrument is a limitation of the Prophet X, not just the sample preparation tool. In terms of number of samples per instrument, however, PXToolkit allows every key to have up to 20 velocity layers, and each velocity layer can have 20 round robin samples. Theoretically, an instrument could consist of 51,200 samples.
I haven’t tried anything quite that packed, but according to Sequential’s documentation it should be supported. I have built instruments far in excess of 128 samples and they work just fine.