أَمَّا الْجَمْعُ، فَشَرْطُهُ: أَنْ يَكُونَ عَلَى صِيغَةِ مُنْتَهَى الْجُمُوعِ
Translation: As for plurality, its condition is that it have the ultimate broken-plural form.
أَمَّا: particle of detail and condition. الْجَمْعُ: nominative topic. فَـ: connective in the response. شَرْطُهُ: subject of the next nominal clause, with شَرْطُ as muḍāf and هُ as muḍāf ilayh. أَنْ يَكُونَ ...: interpreted infinitival clause functioning as its predicate.
This parsing is an added teaching explanation of the source wording.