The minimum capital that is required to trade effectively and compound profit is 50$; anything below that means that you are literally wasting your own calls, signals; because without a good capital; the only way to make money is if you are making over 100% profit which is not sustainable. Your profit is someone else trade capital; so i implore us to twice your trading capital.
I have experiences of trading with below $10 and making more than 20% of profits. In my opinion, your capital is not a big criteria for your success in trading but your technical and money management capabilities. You may argue like to withstand against negative markets, we must need big capital but when your technical analysis is too good then you may enter at the perfect entry level from where market may not show negative directions.
Trading must be a game of skills and capital and all other things are playing just supporting roles. I agree without capital, we cannot get into trade but having very big capital in no way assure you profits; yeah, still you must need good skills.