NeoArkadiaIt 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