If you are looking for a specific collection branded as "21 albums," it generally refers to his core studio catalog or the famous "21 Black Jack" compilation that EMI released to highlight his massive string of hits.
(1993): A return to high-production pop and R&B, featuring "Suave". Nada Es Igual (1996). luis miguel discograf%C3%ADa completa 21 albums
Helpful clue: These are the "kid years"—fun, upbeat, and hard to find today, but essential for collectors. If you are looking for a specific collection