You have a rebate cap of R1,500 while referral rewards are uncapped. There is no limit to the number of people you can refer and so there is no limit to the number of rewards you can receive.
Your rebate cap will be based on a reference currency (ZAR) in which rebates that you receive are calculated on the day of the underlying trade, based on the prevailing exchange rate. Once your rebate sums up to a cap in the reference currency, the cap will be activated and the rebate will stop.
You receive a rebate of 0.001 BTC. That day the exchange rate between BTC and ZAR is R95,000 per BTC. Therefore the contribution to your cap is R95.00 (0.001 BTC x R95,000). The next day you do another trade and receive a rebate of 0.009BTC. The exchange rate has moved to R97,000 that day. Therefore the contribution of this trade to your cap is R87.30 (0.0009 BTC x R97,000). Your total rebate at this point is therefore R182.30 (R95.00 + R87.30). Once this total rebate has reached R1,500, your cap will have been reached.