How many times can a PPF account be extended after maturity?

A Public Provident Fund (PPF) has a 15-year lock-in in India, but after maturity, it can be extended indefinitely in 5-year blocks. This can be done by submitting Form 4 within one year of maturity. One can extend their PPF account either by continuing to deposit money and earn interest, or without contributions, where the existing balance keeps earning interest.