You set a percentage of the deposit, usually 0.5 to 6%, and always make trades for this amount.
If you are a beginner, stick to 0.5 – 1% of the original deposit. If you already have trading experience, you can make trades between 2 – 6% of the deposit.
One important thing is to avoid making trades of 10% or more of your deposit. If in one second, you lose 10% or 20% of the original amount on your account, it becomes hard to recover from the loss.