Adipic acid contains two carboxyl groups (-COOH), each of which can participate in a condensation reaction with glycerol. In a condensation reaction, each carboxyl group reacts with one hydroxyl group from glycerol, forming an ester linkage and releasing water.
Since adipic acid has two carboxyl groups, its functionality for this reaction is 2.
Thus, the functionality of adipic acid is:
\[
\boxed{2}
\]