There is a spec for a number of invalid blocks to be labeled as such at the factory. According to the Samsung datasheet specs for the 32MB NAND flash for the RCM3900, part number K9F5608U0D:
1. The device may include invalid blocks when first shipped. Additional invalid blocks may develop while being used. The number of valid blocks is presented
with both cases of invalid blocks considered. Invalid blocks are defined as blocks that contain one or more bad bits. Do not erase or program
factory-marked bad blocks.
2. The 1st block, which is placed on 00h block address, is guaranteed to be a valid block.
3. Minimum 1004 valid blocks are guaranteed for each contiguous 128Mb memory space.
Jun 12, 2019