spell slots when multiclassing: Spells in dnd 5e – Comprehensive Spellcasting Guide. Warlock Spells 5e: Guide to the Best Warlock Spells - RPGBOT. Known and Prepared Spells – A Simple D&D Spellcasting Guide. Multiclass Spellcasting Calculator: Dungeons and Dragons 5e.
However, even if you get access to a 9th level spell
slot, multiclassing won't necessarily give you access to a 9th level spell.
For example, when you’re a level 5 Warlock, you have two level 3
spell slots. To cast the level 1 spell Witch Bolt, you must spend one of those
slots, and you cast it as a level 3 spell.
When you cast this spell using a spell slot of 2nd level or higher, the spell creates one more dart for each slot level above 1st.
Multiclassing rules say spell slots would be 1/3 of the Rogue levels (3) and all the Wizard levels (3) for a total of spellcaster level 6.