On a side note, I know very little of multiclassing in the 5th edition.
Maybe someone can tell me what type of breakdown in levels is usually considered particularly good? Let's say if a level cap is 12 or 14.
One of the most important things to note is that ASIs are based on class
level, not character level. So multiclassing into a second class at levels 4, 8, or 12 means you are (at least temporarily) sacrificing an ASI/feat. Similarly, you probably want to get to level 5 in your main class because level 5 represents a significant power spike in most classes - e.g., 3rd level spells, Extra Attack, Uncanny Dodge, Stunning Strike (cough cough
Larian please let us test level 5 in BG3).
Starting as a 1st level [Class] vs multiclassing INTO [Class] gives you different benefits. E.g., starting with Fighter gives you Heavy Armor Proficiency, while multiclassing into it doesn't. Additionally, besides Ranger/Rogue/Bard, you only get the skill proficiencies of the class you start in. This is why most caster/fighter multiclassed characters start out as a Fighter.
Fighter 2 is great for most main classes b/c 1st level gets you Second Wind, a Fighting Style, and armor proficiencies; and 2nd level gets you Action Surge.
...that's all I got. I don't multiclass often.