How much blood can a spider drink?

That would depend on the size of the spider.

Spiders do not actually drink blood, they capture their prey then inject it with venom from their fangs. This venom paralyses and kills their prey BUT it also contains digestive enzymes that turn the inside body parts of their prey into a sort of soup. The spider then inserts a sucker and drinks the soup leaving the outside (normally an insect exoskeleton) of the prey as an empty husk.