“The earth abideth forever,” Ecclesiastes 1:4. From this we know that the literal earth is never going to be destroyed.  

“There shall come in the last days scoffers, walking after their own lusts (plans).”  II Peter 3:3  This verse informs us that there will be people on this earth in those last days.

“The Lord is coming with thousands upon thousands of His holy ones to judge everyone, and to convict all of them of all the ungodly acts they have committed in their ungodliness, and of all the defiant words ungodly sinners have spoken against Him,”  Jude 15 (NIV).

“And He shall turn and reconcile the hearts of the (estranged) fathers to the (ungodly) children, and the hearts of the (rebellious) children to (the piety of) their fathers (a reconciliation produced by repentance of the ungodly).” Malachi 4:6   (Amplified Bible)

“For this is the covenant that I will make with the house of Israel (and eventually all mankind) after those days (the last days), declares the Lord: I will put my laws into their minds, and write them on their hearts, and I will be their God, and they shall be my people. And they shall not teach, each one his neighbor and each one his brother, saying, ‘Know the Lord,’ for they shall all know me, from the least of them to the greatest. For I will be merciful toward their iniquities, and I will remember their sins no more.” Hebrew 8:10-12.  (Italicized words are added explanations.)

How many people?  There will be many, many people upon the earth–even children–in those last days.  We are not told an exact number, but enough to require thousands of holy ones to help bring them into reconciliation with God, preparing them for the Restitution blessings of His glorious Kingdom on earth. (See Acts 3:19-21.)