There were 38 Missingnos and each one was a cut pokemon, they'd later be added in G/S/C but narrowing them down has been tough (most agree that the likes of Donphan and Ampharos were in as them being cut explains the....... odd choices in the original Safari Park, as well as ice types like Delibird, Swinub and Poliswine, would have meant Seafoam Islands would have more to offer than Seal) and many think the pokemon that appeared in the anime early like Ho-oh and Togepi were meant to be in it originally, hell, one of the original Developers confirmed that there was meant to be more than the original 151 in Pokemon Red, but they had to be removed for tech and budget reasons....