What do you do when a cat throws up too much?

It depends. If its only once, try watching and waiting. If it's more than once, take your cat to the vet ASAP. Cats can get dehydrated quickly, and they are VERY GOOD at hiding when they don't feel well.

Cats also sometimes throw up because a hairball is stuck and will not come up. Try to prevent hairballs and brush them every day.