Question: How many Rakaats should one read before the Fardh of Esha?

Answer: It is Mustahab to perform four Rakaats before the Fardh of Esha. [Quduri Page 99]

The four Rakaats are not mentioned specifically in a Hadith. Proof is taken from the Hadith
بين كل اذانين صلاة لمن شاء ( صحيح البخاري ج١ص١٧٤)
The Messenger Sallallahu Alaihi Wasallam said; “Between the Azaan and Iqamah there is a Salaah.”

Imam Abu Hanifa is of the opinion that “Salaah” in its complete form and quality is four Rakaats. [Munyatul Musalli Lil Halbi]

And Allah Ta’ala Knows Best