I think it's depend on your whole assets, it's would be 0.05 or 0.5 depend on how rich are you. Or maybe you could try to do the signature campaign and keep all the rewards for trading.
There is no amount that is minimum. Yeah some money beyond the thresh hold point is important to start investing. Even you start it with the minimum amount; it will help you learn about the market behavior and the market response. Besides, you would be able to apply and use your strategies while dealing or trading. It will also help you modify and observe your response to the market crash. Also you could earn some money.