Cultures come in packets that are labelled with how many litres of milk they will inoculate. Our yoghurt cultures come in a packet that will inoculate 100 litres. Cheese cultures can come in packets that inoculate 100, 250 or more litres.
How much is in the packet will vary from culture to culture and even batch to batch of the same culture. Remember, the culture is alive, so it will not be the same for every batch that is grown.
Culture is not measured by weight or volume, but rather by its rate of activity. The activity rate of a culture is most easily determined by measuring the acid production or decrease in pH of the milk it is added to. To have an easily understandable system, the growing conditions are standardised by having the same medium, temperature and time. For example, the use of a standard weight/volume percentage concentration (w/v) reconstituted skimmed milk, at 30oC for six hours. The manufacturer does all of this for you and puts the correct amount of culture in the packet for the amount of milk it will inoculate.
A culture could have one teaspoon for 100 liters, and then next time you get the same culture, or a different culture, it could be one and a half teaspoons for the same amount of milk, or even as little as half a teaspoon. No matter how much culture is in the packet, it will be enough for the number of litres listed on the packet, be in 100, 250 or 1000 litres.
For more information please read How much culture do I use?