You define a percentage of your deposit, usually between 0.5% to 6%, and make every trade with this amount. It is recommended not to exceed 5-6% in any case for any of the open positions. For example, if you trade with 1% of your deposit, do not open more than six trades simultaneously.
If you are a beginner, stick to 0.5-1% of your deposit amount. If you are already an experienced trader, you might make trades with 2% to 6% of your deposit.
The important thing is to avoid making trades with 10% or more of your deposit. It will be challenging to recover from losses if you lose 10% or 20% of your deposit in just a second.