In bulk, monodisperse colloidal particles crystallize into close-packed structures upon reaching a critical packing fraction. However, when confined in a finite space, the confinement geometry, as well as the number of particles, gives rise to complex crystallization behavior and structures. We experimentally investigate the assembly behavior of sub-micron colloidal particles in the spherical emulsion droplets produced by microfluidics. We demonstrated that such confined self-assembly systems follow a magic number effect and prefer closed-shell structure, in analogy to the formation of atomic clusters or atom nuclei. With the right number of particles in the droplet, the entropy-driven process yields diverse icosahedral structures with precision and suprising structural complexity. Otherwise, defect particles accumulate in a defined wedge region instead of distributing homogeneously to the cluster. This wedge reduces the free energy penalty associated with inevitable structural disorder. We work in close collaboration with the groups of Prof. Michael Engel and Prof. Erdman Spiecker to analyse the 3D structure with advanced microscopy methods (Spiecker group) and to study the kinetics and thermodynamics properties of these confined colloidal clusters in simulation (Engel group).