In the leaderboard settings, you are able to set the number of records to display. Go to Edit Widget / Dimensions / Max rows to display. This field lets you set how many records the leaderboard will store. You can go as high as 100 records.

If the data has been collected, can the lost data be retrieved？

If you want to store all the data, consider using the Data Cloud or Sheets widgets. A leaderboard is by definition a list of leaders, not the entire list of records. If you need to, you can use both the Data Cloud to store all records and the Leaderboard to display the top records.

The data is important, the background of the data lost in the ranking list cannot be recovered？

Yes, the rest of the data is not kept anywhere, it only saves the amount of rows you set in the widget settings.