A single-gene trait that has two alleles and that shows a simple dominant-recessive pattern will result in

Two phenotypes.

Explanation:

When there is a single gene trait and it shows a simple dominant-recessive pattern then it will result in two phenotypes. As the name suggests, a single gene trait is a trait which is controlled by only one gene.

For example, consider a trait Tallness which is a single gene trait and the two alleles for this gene are T and t where T is dominant and t is recessive. The different phenotypes in different allelic combinations has been shown below -

TT - Tall

Tt - Tall

tt - dwarf

Thus we see here that only two phenotypes Tall and dwarf are present for the all possible genotypes.