Multiclassing is the process of taking different classes in order to gain an advantage in one area of your game. For example, a paladin could multiclass into a ranger in order to gain better tracking abilities, or a wizard could multiclass into an fighter in order to gain better melee abilities.
The assumption is that multiclassing will give you more spell slots than if you had just taken one class.
There are a few factors that can affect this assumption. First, multiclassing can give you access to spells that you would not have been able to cast if you had only taken one class.
For example, a cleric could multiclass into a warlock in order to gain the ability to cast spells of the dark magic school. This gives the warlock four extra spell slots, which would have otherwise been unavailable to them.
Second, multiclassing can give you access to spells that would not have been available to you if you had taken two different classes. For example, a paladin could multiclass into a druid in order to gain access to the wild shape ability.
This gives the druid four extra spell slots, which would have otherwise been unavailable to them.
Finally, multiclassing can give you access to spells that would not have been available to you if you had taken three different classes. For example, a paladin could multiclass into a warlock, mage, and bard in order to gain access to the spellbook ability and all three of their respective spell slots (for a total of eight).
All three of these points mean that it is difficult to say definitively whether multiclassing will give you more spell slots than just taking one class. The most important thing is to think about what kind of advantages you hope to gain from multiclassing and choose the class that best suits those goals.
