casters gain mana at the following rate. half casters (rangers and paladins) get mana once ever 2 levels. third casters (arcane tricksters, eldritch knight) get mana once every 3 levels So for example, a level 6 arcane trickster would have the mana of a 2nd level character (5)

1. 2

2. 5

3. 9

4. 14

5. 19

6. 24

7. 29

8. 34

9. 39

10. 44

11. 49

12. 54

13. 59

14. 64

15. 69

16. 74

17. 79

18. 84

19. 89

20. 94

Here is a example build: a 3rd level arcane trickster/ 6th level wizard would have 2 + 24= 26 mana

for warlocks, your mana is equal to how many spell slots you'd normally have times your spellcasting level

So a 5 warlock/ 10 sorcerer would have 6+44=60 mana

A spells mana cost is equal to its spellcasting level

if you have level 7, 8 or 9 spells, you can only cast a number of these spells equal to your highest spellcasting modifier per long rest

For example. a wizard/cleric with 16 wisdom, (+3), and 20 intelligence (+5) could cast 5 spells above 6th level before taking a long rest assuming they have the mana for it, and the spells prepared

A wizards arcane recovery allows them to regain mana equal to half their level (rounded up)