How much should I start with to trade forex in South Africa without overexposing myself?

I think you should only trade with money you can afford to lose. I know this is easier said than done but that is the reality.

Only you can determine your risk appetite but $50 is a good amount to start trading with, as it will allow you to place your stop loss not too close to your entry and give the market time to move in your direction.

Note that when you trade with a small account balance, it makes you place your stop loss too close to your entry and you get stopped out easily/repeatedly.

You need to place your stop loss at a price that is not too predictable because professional traders look to place their entry where inexperienced traders will place their stops.

R50 is okay to start trading with but keep your lot sizes at 0.01 for each trade so you dont blow your account quickly. Also set a tight stop loss for each trade and avoid the temptation to adjust the stop loss to accomodate more losses. if you make a loss, do not try to recover it that day, wait till the next day.