Yes, you can link multiple Demat accounts (BO IDs) to a single trading account. This allows you to buy/sell using one trading account while holding securities in different Demat accounts.

Yes. Your BO ID links all your Demat holdings across securities like equities, bonds, ETFs, etc., and is used by back-office tools and apps to generate portfolio reports.

There is no regulatory cap. You can open multiple Demat accounts (and hence multiple BO IDs) with different DPs/brokers, provided you comply with KYC norms.