Answer:
A counterexample is sufficient, as long as it is well built and complete.
Explanation:
A counterexample is a statement used to show that a previously spoken statement is false.
The counterexample must be direct, clear, simple, well constructed and concise to be effective. If the counterexample has all of these characteristics, only one counterexample is sufficient to show that a conditional statement is flawed.
As an example of this, we can use a square as a counterexample of a statement that states that every rectangle has 4 equal sides.