Suppose only one gene with 2 alleles determines a particular trait but the alleles are codominant. how mant different phenotypes could exist for this trait

The answer is one trait. In Codominant alleles, neither allele is dominant over the other. Both alleles are expressed equally hence no recessive or dominant phenotypes. An example of that of fur color in rabbits where one parent with black fur color and the other with white fur color will produce offspring of black and white fur color. This should not be confused with incomplete dominance whre the two alleles blend in phenotype. Example of incomplete dominance is a parent with straight hair in humans and another with curled hair will produce wavy hair offsprings.