After everyone has entered, there is space left
A description of what happens when the Garden is not filled.
يَبْقَى مِنَ الْجَنَّةِ مَا شَاءَ اللَّهُ أَنْ يَبْقَى ثُمَّ يُنْشِئُ اللَّهُ تَعَالَى لَهَا خَلْقًا مِمَّا يَشَاءُ
There would be left some space in Paradise as Allah would wish, and then Allah would create a people for it.
The full narration
Anas reported Allah's Apostle (ﷺ) as saying:There would be left some space in Paradise as Allah would like that to be left. Then Allah would create another creation as He would like
Source
Ṣaḥīḥ Muslim · 7180
Ṣaḥīḥ
- Agreed upon as authentic (al-Bukhārī / Muslim)