13 oz of coffee equals how many 6 oz cups?

This clearly depends on the size of the cups. For a 6 ounce cup you would need 36 ounces of water which is two and a quarter pints US or One and three quarter pints UK

16 oz is a US pint

20 oz is a UK pint

If you are making espresso the cup size is much smaller but you probably wouldn't call that "brewing" coffee, which sounds like having it sitting in a container you are going to pour from.