This wardrobe is so big that I don't believe only one person assembled it.

HAVE (4 words)

This wardrobe is so big that it together by only one person.

can't have been assembled

Explanation:

Modal verbs used to express a certain level of possibility or necessity can be quite tricky.

Can't have been is a modal verb phrase used to express one's opinion regarding the level of uncertainty of a near past event.

From the original sentence we see that the person expresses his disbelief regarding the wardrobe being assembled by only one person.

If we presume that the wardrobe was relatively recently assembled, than it would meet the conditions for using this modal verb phrase.

Although it is unknown from the context of the sentence, the wardrobe might have been assembled long time ago. That could be the indication for using another modal verb phrase - couldn't have been.

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