Hazrat Umar bin Abdul Aziz says that he will say two takbirs.

Ibrahim Hanafi reports that I asked Hazrat Ibn Sirin about a person who joins the congregation while the Imam is in the state of bowing (ruku‘). He said: He should say the opening takbir (takbir tahrima) to enter the prayer, and then say another takbir and join the bowing. If he does not do so, his prayer will not be valid.

Hazrat Abu Abdur Rahman says that one takbeer will be said to join the prayer and one takbeer will be said for bowing (ruku).

Hazrat Abu Abdur Rahman says that he will say two takbirs.