Is it true that the humanity of manifest destiny was that America was destined to comprise of 50 states?

No, the concept of manifest destiny in American history refers to the belief in the 19th century that it was the United States' destiny to expand across the continent from the Atlantic to the Pacific Ocean. It did not specifically entail that America was destined to have exactly 50 states. The number of states in the US has changed over time as new territories were acquired and admitted into the union.