If there are fifty Pokémon on the Friend Board that have not been sent to the game yet, the Island of Dreams cannot be visited. Due to players only being able to have one Pokémon from the Tree of Dreams on the Friend Board at a time, this would require the player to have 49 Pokémon from Pokémon Global Link promotions on the Friend Board that were not sent to the Entree Forest; therefore, this circumstance is extremely unlikely to occur. At one time before the international release of the Dream World, only fifty Pokémon could be transferred from a Global Link account; this was removed in 2010.
Yes, it was true until 2010. So now, you can get as many Pokemon as you want.