By the time you notice thinning, you've likely lost half of it.
You don't notice hair loss from the first strand.
The human eye doesn't register thinning in an area until it has lost roughly half its original density. So the moment you look in the mirror and think you're starting to thin, you have probably already lost half the hair in that area.
That is the main reason most people act late.
If you have noticed a change, this is not the moment to watch and wait — it is the moment to get assessed. Treating loss early protects hair you still have, and that is easier and cheaper than transplanting to replace hair that is already gone.
What does your case actually need?
The video explains the general rule. The number that applies to you is settled after an examination — the grafts your case actually needs, not a figure quoted before we have seen you.