Depending at which level you are defining an allele. As @Adhish described, at the molecular level there is no theoretical limit on the number of alleles of a given gene. The presence or not of more than 2 alleles depends on evolutionary factors such as population size, fitness change, genetic drift, etc. Hardy-Weinberg Equilibrium with two alleles is easier to understand but is possible to extend the genotype predictions for multiple alleles in the equilibrium. Also remember that the Hardy-Weinberg Equilibrium is theoretical and requires assumptions that are never really met in nature, so is extremely hard for multi allele systems to ever reach the equilibrium.