There are no hard rules on this, but it is common for traders to aim for around 200 trades to constitute a complete backtest. You probably should not go with less than that, but you can certainly add more trades into your backtests if you wish. The more data you have to work with, the more accurate the portrait of your system’s effectiveness will be. The nice thing about backtesting is how rapidly you can move through it. You will be surprised by just how quickly you can complete hundreds of trades on paper. You can cover years of time on your charts in a matter of days. That means you can gather a lot more data rapidly with backtesting than you can with demo testing (that said, demo testing is a critical part of learning to trade).