1 they are supper small
2 they will prob cost to much and where would you even get 40 leeches
3 anticoagulant might kill the medical leeches you see in videos on you tube in video only suck small amounts of blood not near the 20-25% you need for this to work if you used 40 who knows what might happen
4 its prob hard to get large amounts of blood fast from a non vein and whats you going to do let the leeches bite same vein 5-10 times best case
I got around to actually trying it, bought 3 leeches pretty cheap but transport was very expensive.
Turns out that leeches excrete hirutin, an anticoagulant protein that mainly works localized where the leech bites but also has systemic effects.
Yes, the leech will only drain small amounts of blood, up to 50ml if you're lucky, but due to the anti-coagulant you will continue to bleed for at least 3-6 hours, up to 24 hours. For me it is at least 6 hours of continuous bleeding.
So I place two plastic bags on my foot and add a leach and let it bite and suck on me, and then I allow my wound to bleed uninterrupted. That way I can lose a fair amount of blood, I'd say at least around 100ml from the bleeding itself after the leech has left.
I also have drained my own blood with a butterfly needle, which is very easy and obviously extremely cheap.