Step 1: Understanding the Concept
Multiple alleles means a gene has more than two allelic forms in a population.
Step 2: Individual
A diploid individual carries only two alleles, one on each homologous chromosome, for that character.
Step 3: Example
In ABO blood groups there are three alleles in the population but each person has two.
Final Answer:
One individual can have only two alleles.
\[ \boxed{\text{(C) }\text{two}} \]