Learn Arabic (Modern Standard): كم من فلفل حار أضافت الطباخة!

You are thinking of the regular question word كم (interrogative kam), which asks "how many?" and is followed directly by a singular noun. Here, we are using the exclamatory كم (kam al-khabariyya) to mean "Wow, how much/many!". This type of كم is very often followed by the preposition من (of), causing the noun after it to take the genitive case (an i or in sound).

What case ending would فلفل حار have when read aloud?

Because of the preposition من, the noun فلفل (pepper) takes the genitive case, giving it a kasratayn ending (فلفلٍ - fulfulin). Since حار (hot/spicy) is an adjective describing the pepper, it must match the noun's case exactly, becoming حارٍ (Haarin). So you would pronounce it: kam min fulfulin Haarin!

What is the root of the verb أضافت (added)?

The root is ض - ي - ف. This is a Form IV verb, which often carries a causative meaning (making something happen). Because the middle root letter is weak (a "hollow" root), it changes to an alif in the past tense, making the base "he" form أضاف (he added). We simply add the ـت suffix for "she" to match the female cook, giving us أضافت.

Why is the subject الطباخة (the cook) at the very end of the sentence?