To be quite frank, as little as $5 will be enough. There are a few online brokers, especially those that utilize Robo-advisors, that will allow you to open an account with almost nothing.
However, most proper online brokerages will require at least a few hundred dollars of deposit at the beginning. This is not bad, as building a proper portfolio that will allow you to achieve sufficient diversification will require at least a few thousand dollars. If you do not have that much cash at hand right now, you might be better off investing in funds until you do.
What’s more important is that if you deposit enough money, you can avail promotions that will make investing very easy for you. We discuss some of these promotions later on.