How much scoby do I need to use when re-culturing?

You will need 300 mL (10 US fl oz) of brew from your previous batch along with a scoby. You do not need a whole scoby, if you want to make multiple batches you can cut a scoby in half and just use that. The important piece is ensuring you have 300 mL (10 US fl oz) of starter culture.