Ask any prop traders.. the answer is none. If your trading system has clear rules, your trading system has edge, your position sizing per trade is not going to ruin your capital and you are aware of the law of large numbers, the answer should always be none.
Forex trading is always a struggle not only with the market, but also with yourself. After all, you can have a fairly high-quality trading strategy, but if you are undisciplined, passionate, do not know how to wait, are often visited by fear or greed, then you will turn any strategy into a loss-making one and will not earn anything on Forex. Therefore, it is very important to be able to control your emotions and be psychologically balanced when trading, this is a fairly significant factor in trading.