Under standard circumstances, how much do I need to invest and where to have a regular income of about 20,000 INR per month?

This depends on your current life cycle stage and income tax slab. The options are:

1. Invest in Equity Mutual Funds or Balanced Mutual Funds that give a dividend payout. Note that this is subject to a dividend distribution tax of 10%. Suggested investment amount: Rs 35 to 40 lakhs
2. A variant is to do a Systematic Withdrawal Plan (SWP) from your equity mutual fund after 1 year. In such a case there is a long term capital gains tax, but only after profits you make on those withdrawals cross one lakh. Suggested investment amount: Rs 20 to 25 lakhs
3. Invest in Debt Mutual Funds that give a dividend payout. Note that this is subject to a dividend distribution tax of 25%. Not recommended for most people.
4. A variant is to do a Systematic Withdrawal Plan (SWP) from your equity mutual fund after 3 years. In such a case there is a long term capital gains tax, but with indexation benefit the tax paid could be very negligible. Suggested investment amount: Rs 30 to 35 lakhs

These are very high level numbers and you need to speak to an advisor with your exact numbers. As you can see they vary based on risk level, taxation, dividend mode etc.