A druid can prepare a number of spells equal to their Wisdom modifier + their druid level (minimum one). For example, a 5th-level with a +3 Wisdom modifier can prepare 8 leveled spells. Druids can change their prepared spells when they finish a long rest, taking 1 minute per spell level for each spell.

Druids can only prepare spells for which they have the spell slots. To continue the example, a 5th-level druid only has access to 1st-, 2nd-, and 3rd-level druid spells. When selecting their prepared spells at the end of a long rest, they can choose from any of these.

They can choose to prepare all 1st-level spells or choose from a range of different leveled spells. However, it is wise to have a range of leveled spells to accomodate all available spell slots. Lower-level spells can be cast using higher-level spell slots, but not vice versa.

Cantrips do not count as prepared spells. A druid learns cantrips at certain levels and cannot change them at the end of a long rest. However, they can exchange one druid cantrip for another whenver they reach a druid level that grants an Ability Score Improvement (4, 8, 12, 16, 19).

Player’s Handbook, page 66 + 201; Tasha’s Cauldron of Everything, page 36

Druid Spell Slots per Level

Level	Cantrips Known	1st	2nd	3rd	4th	5th	6th	7th	8th	9th
1st	2	2	–	–	–	–	–	–	–	–
2nd	2	3	–	–	–	–	–	–	–	–
3nd	2	4	2	–	–	–	–	–	–	–
4th	3	4	3	–	–	–	–	–	–	–
5th	3	4	3	2	–	–	–	–	–	–
6th	3	4	3	3	–	–	–	–	–	–
7th	3	4	3	3	1	–	–	–	–	–
8th	3	4	3	3	2	–	–	–	–	–
9th	3	4	3	3	3	1	–	–	–	–
10th	4	4	3	3	3	2	–	–	–	–
11th	4	4	3	3	3	2	1	–	–	–
12th	4	4	3	3	3	2	1	–	–	–
13th	4	4	3	3	3	2	1	1	–	–
14th	4	4	3	3	3	2	1	1	–	–
15th	4	4	3	3	3	2	1	1	1	–
16th	4	4	3	3	3	2	1	1	1	–
17th	4	4	3	3	3	2	1	1	1	1
18th	4	4	3	3	3	3	1	1	1	1
19th	4	4	3	3	3	3	2	1	1	1
20th	4	4	3	3	3	3	2	2	1	1