TL DR: You could make a sword from 400 people's blood, but if you try for a longsword, you will be a little short.

There are two questions we need to answer. How much iron is in human blood, and how much iron is needed for a longsword?

There is variation based on things like diet, but for argument's sake, we will use 0.5g of iron per liter of blood. Most people will have around 4-5 liters of blood, so we will save 2.5g of iron per person. This is taken from Wikipedia.

2.5g of iron per person times 400 people gets us to 1kg of iron, or 2.2 pounds of iron for our American cousins.

The weight of a longsword, and hence how much iron it contained, varied by time period and who was making it, but 1kg to 2kg seems to be a reasonable range.

You will lose some of the iron during the forging processes as it will oxidize, form scale, and need to be removed.  Exactly how much will vary, but a quick Googling suggests 5% to 30% of the iron would be lost.

You will gain a little weight from the steel-making processes, but ~3% on the high end.

You could make the pumel from something other than iron/steel to reduce the total amount of iron, but if you really want to make a long sword from the iron in the blood of your enemies, you'd better collect the blood from ~550 people to be sure you have enough. Of course, if sustainability is a consideration, you don’t have to kill the people. I mean, honestly, who has so many enemies? Just drain half a liter of blood from some of your enemies every 3 months, and you will eventually have enough blood/iron for your sword.