How much should I invest if I want to get $900 passive income on a monthly basis?

$900 monthly would be $10,800 annually. Assuming an estimate of 4% for dividend yield/coupon rate, you will need to invest $270,000. It is important to bear in mind that the higher the rate, the higher the risk of losing your principal. This is in the scenario where there is zero taxation. If you invest in overseas equities/bonds or receive royalties, a tax may be imposed and will reduce your net income.