Hi Krishna,
Systemically, SEBI regulations (Peak margin requirements from Sep 1st 2021 & its effects) make it difficult for clients to take positions with excessive leverage. Which is one line of defense before reaching the default stage. Also the margin requirements closer to expiry that has been set up by us helps reduce defaults to a large extent - What is Zerodha's policy on the physical settlement of equity derivatives on expiry?.
The other line of defense is our RMS team who track and manage the risk as much as possible.
But of course there can be defaults despite this - which can attract legal action as well.