It is clarified that Grandmaster Rares are 100 copies per card, with cards having a Serial Number between 001 and 100, making these some of the rarest cards in the entire game.

Real Life Samples:

Grandmaster Rare

A regular Over Frame

Also, it was previously stated this new rarity would be a Prismatic Secret Rare with a Serial Number. That is no longer the case. The following changes have been announced for the Limit Over Collections:

【Configuration Changes】
● 80 Different Cards (38 Ultra Rares + 42 Super Rares)
※ All cards come in Prismatic Secret Rare & Secret Rare
※ 18 of the Prismatic Secret Rares come in Over Frame Cards (i.e. Extended Art)
※ 18 of the Ultra Rares come in the new rarity “Grandmaster Rare” that comes with a Serial Number.
※ 18 of the Ultra Rares come in Over Frame Cards (i.e. Extended Art)
※ 21 of the Super Rares come in Ultimate Rare
※ 21 of the Super Rares come in Collector’s Rare

Source: YU-GI-OH!.jp