I'm really confused by Multiclassing as a Warlock. Say I put 19 levels into Warlock and I have six spell slots. Simple enough.
The tricky bit is when you think of ways to get more spell slots. The most obvious way is to Multiclass into another spellcasting class - such as Wizard. If you look at the 'Multiclassing' area of the PHB, there's the following extract and a table for calculating spell slots when Multiclassing.
Pact Magic. If you have both the Spellcasting class feature and the Pact Magic class feature from the warlock class, you can use the spell slots you gain from the Pact Magic feature to cast spells you know or have prepared from classes with the Spellcasting class feature, and you can use the spell slots you gain from the Spellcasting class feature to cast warlock spells you know. — P.164 PHB
Thing is, does Multiclassing a level into Wizard increase your spellcasting class to lv.20 and hence by the table give you a massive boost of spell slots which can then be used with Pact Magic?