When such a character gains a level, instead of gaining the next level of his existing class, he may add a level of a new class and add all 1st-level class features of that class to his existing class features. This is known as “multiclassing”.
There is no way to subclass multiple classes of the same class, the PHB does a good job of supporting this view. As Sagatympana said, if you want to brew this yourself, you always have the option of having a “talent tree” or the “choose your function” option.
You can have one class per character level
There is no limit to how many different classes you can select when multiclassing.
1 Paladins Taking the best of everything and excelling at everything
Of all the classes in the latest installment of D&D, the Paladin is perhaps the most powerful of all classes in combat.
Basic Attack Bonus
For classes with a d8 Hit Die, their BAB increases by 3/4 per level. For classes with a d10 or d12 to-hit die, their AP increases by 1 per level (so there is no need to round the AP for these classes). A multi-class character’s base attack bonus will only ever improve with this variant.
Multiclassing allows a character to have two classes at the same time. Multiclassing is chosen at character creation and cannot be added or changed later. As mentioned above, characters with multiple classes cannot access the highest power levels (VIII and IX) of their classes and unlock new power levels more slowly.
Multiple classes? Hi. Yes, you have to choose another class to level up and when you level up afterwards you choose which class you want to level.