Lord Krishna Himself gives the answer in the Bhagavad Gita

Bhagavad Gita manuscript is found in the sixth book of the Mahabharata manuscripts ¿ the Bhisma-parvan. Therein, in the third section, the Gita forms chapters 23¿40, that is 6.3.23 to 6.3.40.The Bhagavad Gita is often preserved and studied on its own, as an independent text with its chapters renumbered from 1 to 18. The Bhagavad Gita manuscripts exist in numerous Indic scripts. These include writing systems that are currently in use, as well as early scripts such as the Sharada script now dormant. Variant manuscripts of the Gita have been found on the Indian subcontinentUnlike the enormous variations in the remaining sections of the surviving Mahabharata manuscripts, the Gita manuscripts show only minor variations and the meaning is the same. According to Gambhirananda, the old manuscripts may have had 745 verses, though he agrees that 700 verses as the generally accepted historic standard. Gambhirananda's view is supported by a few versions of chapter 6.43 of the Mahabharata. These versions state the Gita is a text where "Kesava [Krishna] spoke 620 slokas, Arjuna 57, Samjaya 67, and Dhritarashtra 1", states the Religious Studies and Gita exegesis scholar Robert Minor.