The capital you need depends on the strategy you develop and how that is structured, if you develop for example on Forex you can size your trades small enough to work with a limited capital (using mini or microlots), Dax future requires a higher amount of money and you can size that based on the impact of the largest losing trade of your strategy or on the drawdown, if that would be too much you could for example take trades on miniDax.

To get more into details, if you want to trade "expensive" markets such as Dax future, Silver, Crude and so on you obviously need a certain capitalization (at least around 100k$ I would say - I'm talking about full contracts, not mini, micro, or CFDs) to decently face drawdown periods (for a single strategy that could be cut in half to 50k, but I imagine a portfolio right now). Nevertheless if you plan to work on miniDAX, etc. you can scale down to about 20k. Also on forex there is no lower limit to capital as you can scale down the position to mini or microlots and trade with low risk accordingly.