The exact number of partitions allowed on a drive is fixed by the kernel. So you can find the exact number is the kernel documentation, the last version is maintained here http://kernel.org/pub/linux/docs/device-list/If you have the kernel source installed, you can find your version on your computer at /usr/src/linux/Documentation/devices.txt.

Look at "limit on partition". Find yours. Common SATA number is 31, SCSI is 15, some are less.