Question regarding the number of alleles
The two-allele scenario is often used in genetics teaching because of its simplicity. However, quite a few genes have more than two alleles. Some examples that readily come to mind are:


The ABO gene in humans: This determines ABO blood group, and has six common alleles. Many more rare alleles have been described [1].
The human leukocyte antigen (HLA) genes: These are well-known for their allelic diversity. One of them, HLA-A, alone has over 6,000 alleles [2].




Reference


Seltsam A, Hallensleben M, Kollmann A, Blasczyk R. The nature of diversity and diversification at the ABO locus. Blood 2003; 102(8):3035–3042. https://doi.org/10.1182/blood-2003-03-0955
Robinson J, Halliwell JA, Hayhurst JD, et al. The IPD and IMGT/HLA database: allele variant databases. Nucleic Acids Research 2015; 43:D423-431. http://hla.alleles.org/nomenclature/stats.html