It depends. The blood they collect for storeage isn't just out of the cord it is also out of the placenta. What they do is clamp and cut the cord as soon as the baby is born then while the placenta is still attached they put a needle into the umbilical vein and let placenta pump (using your heart) blood down the cord and into their collection bag. you should not be given Syntocinon until the collection is complete as it makes the placenta seperate from the uterine wall and then the blood flow stops. This is how different women end up with different amounts of blood as some inexperienced practitioners either try and collect from the umbilical artieries instead of the vein or give synto then try to collect. I'm not sure of the amount but I know if there is under a certain amount then it gets rejected by the collection agency. So if you're plannning on having it collected make sure you Dr/ Midwife rings the collection agency and gets strict instructions if not training before they collect because it is a very delicate thing to do and if not done perfectly the blood and cells are damaged rendering it useless.
Also be aware that it is not cheap- about $1000 a year to store for 18 years plus collection and transport costs. The free collections that some big hospitals are now doing are not stored exclusively for you baby's use, these stores are donated to anyone with a matching tissue type needing the blood.