Based on your TF of choice, I would recommend you backtest any currency pair you find your strategy works best, keep the trading journal record so that your result is unbiased and then choose a few pair from the pairs you backtested so that you can maximise your strategy, once you gain more experience then increase a pair at a time. Go slowly but surely!
I usually prefer trading on major currencies such as EUR/USD, USD/CHF due to their least volatility.
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