Yes, you can entangle more than two particles or qubits, it has been done many times already. However, it becomes increasingly difficult with higher numbers of objects. Entangling particles or qubits "by hand" up to macroscopic scales is unlikely, at least for now because it's a huge number of things.

On the other hand, superconductivity is an entangled state in some sense, and definitely reaches macroscopic scales.

Angular momentum quantization isn’t a postulate, it shows up when solving Schrodinger’s equation for the hydrogen atom.