Lord Parashu Rama sanitized earth by killing kshatirya generations for 21 times. From what Dharma calls that the one who is humble and does not raise a weapon he should be granted sharanagati, & due to this Lord Parshuram would have been merciful to the Kshatriyas who surrendered to him. So according to this formulation of Dharma how many Kshatriyas were left alive after each wipe-off for kshatriyas?.(Lord Parashu Rama did 21 such wipe-offs). This is the reason for doing 21 wipe-off, killing 21 generations of Kshatriyas.
In this chapter of the Shanti Parva of the Mahabharata, Bhumidevi goddess of the Earth tells the sage Kashyapa about the Kshatriyas who have survived:
There are, O, regenerate one, some foremost of Kshatriyas concealed by me among women. They were born in the race of Haihayas. Let them, O sage, protect me. There is another person of Puru's race, viz., Viduratha's son, O puissant one, who has been brought up among bears in the Rikshavat mountains. Another, viz., the son of Saudasa, has been protected, through compassion, by Parasara of immeasurable energy and ever engaged in sacrifices. Though born in one of the regenerate orders, yet like a Sudra he does everything for that Rishi and has, therefore, been named Sarvakarman (servant of all work). Sivi's son of great energy, viz., Gopati by name, has been brought up in the forest among kine. Let him, O sage, protect me. Pratardana's son, named Vatsa of great might, has been brought up among calves in a cowpen. Let that one of the royal order protect me. Dadhivahana's grandson and Diviratha's son was concealed and protected on the banks of Ganga by the sage Gautama. His name is Vrihadratha. Possessed of great energy and adorned with numerous blessed qualities, that blessed prince has been protected by wolves and the mountains of Gridhrakuta. Many Kshatriyas belonging to the race of Maratta have been protected. Equal unto the lord of Maruts in energy, they have been brought up by Ocean. These children of the Kshatriya order have been heard of as existing in different places. They are living among artisans and goldsmiths.
As I discuss in this answer, following Bhumidevi's instructions Kashyapa, who had been given the Earth by Parashurama, gave the Earth to those Kshatriyas.