I don't know based on the rulebook. The provided context explains how to gain, spend, and keep Energy cubes (see page 2 and page 4), but it does not specify any maximum limit to the number of Energy cubes a Monster can hold. The rulebook only mentions a maximum for Life Points (10, unless a keep card says otherwise, see page 6), but no such rule is stated for Energy cubes.

To answer definitively, the rulebook would need to explicitly state a maximum limit for Energy cubes, which it does not in the provided context.

Source: Core Rulebook (pp. 2, 3, 4, 6)