In a diploid population of size N, total number of alleles is
Diploid organisms carry two homologous copies of each autosome, each bearing one allele per locus. In census population of N individuals, each contributes two gene copies for any autosomal gene, so total allele count in gene pool equals 2 multiplied by N. This denominator is used to convert allele counts to frequencies; for example 100 individuals yield 200 alleles. For X-linked loci counts differ due to hemizygous males. Concept extends to ploidy generalizations: total allele number equals ploidy level times population size, essential for sampling variance in drift models.
Ref: Hartl & Clark, Principles of Population Genetics, 4th ed., Chapter 2: Allele Counting; Griffiths, Chapter 20: Diploid Totals