A 50 kg healthy person, with a total blood volume of about 4000 ml. About 80% of the circulating blood in the human body is running, and the other 20% is reserved.
Donate blood at most 400 ml at a time, accounting for only one tenth of the total blood volume. There will be no problem and no special supplement is needed.
As long as you have enough food and are not nervous before donating blood, you will generally not feel dizzy.