i know "tu" is not required for this sentence, but is "Tu quinque crustula habes" actually wrong?
It shouldn't be. I had a word-select answer, though, and 'tu' was not offered, so I don't know if they still mark it incorrect (I assume they marked it incorrect since you are asking).
Isn't "crustula" singular?
No, crustulum is the nominative, accusative, and vocative singular.
crustula is the nominative, accusative, and vocative plural.
It's a second declension neuter, if that helps.
Doesn't it seem like it would help to have the declension/conjugation of the word being hovered over listed in the Latin word hints?