Even though the human genome contains roughly 20,000 protein-coding genes, it has the capacity to produce several hundred thousand gene products because there are several mechanisms at play here:
1. Transcription and splicing: mRNA variants of a single gene are produced because of the splicing. These mRNAs then get translated to proteins that are different from each other.
2. Epigenetics: The genes have special markers on them which vary from cell to cell in an individual. The proteins synthesised by such genes are different in nature from one another.
3. Dimer and polymer proteins: Proteins after getting synthesised merge to form other special proteins that exist in the form of a pair or a long chain of varying length.