There is an extra sloka because of the inclusion of Arjuna’s question in the beginning of chapter 13.
This was done to add continuity and is omitted in some texts.
As per the Bhishma Parva of the Mahabharata, the Gita has 745 verses
(Krishna has 620 shlokas, Arjuna 57, Sanjaya 67, and Dhritarashtra 1 -
making a total of 745 verses).
However, the most popular
versions, mostly identical with the one commentated by Sri Adi
Sankaracharya, which is said to have 700 verses (Krishna - 574, Arjuna -
84, Sanjaya - 41, and Dhritarashtra - 1 - Total = 700).
The Kashmiri version, commentated by Sri Abhinava Gupta, has 745 slokas.