The recovered spell slots must have a combined level that is less than or equal to half your Druid level (rounded up).
For example:
When a Level 3 Druid uses Natural Recovery:
The highest Level of Spell Slots that can be recovered is: 3 ÷ 2 = 1.5 ≈ 2
The total number of Spell Slots that can be recovered is: two Level 1 Spell Slotsorone Level 2 Spell Slot.
When a Level 5 Druid uses Natural Recovery:
The highest Level of Spell Slots that can be recovered is: 5 ÷ 2 = 2.5 ≈ 3
The total number of Spell Slots that can be recovered is: three Level 1 Spell Slotsorone Level 2 Spell Slot and one Level 1 Spell Slotorone Level 3 Spell Slot.