In this classification, simple (mufrad) means a term treated as one subject or model, even if several words are used to express it. Composite (murakkab) means a scene or configuration formed from interacting details.
Comparing one thing with musk for its fragrance involves individual terms. Comparing dust, flashing swords, a dark night and falling stars depends on a constructed scene.
Thus word count is not the test. Identify whether the comparison depends on a single quality or on a structured combination of features.