Can you tell me in which year should I start interning at law firms and how many internships should I do till PPOs and Day Zero?

Earlier the better.

Quality matter more, long term internship are a big green flag.

5 long term internship with boutique firms in five years of your law school have way more value than 10 1-month tier-1 internships.

Pick a boutique firm, try to convert your internship into long term or even become a paralegal if possible.

Target tier-1 firm only with the view of displaying your performance.