How much of a culture do you need per batch approximately?
Which culture, p roqueforti? Depends on the strength you need in the final cheese, and method of inoculation, but generally, you need about 2.0 x 10^9 - 4.0 x 10^9 spores, with a germination rate of at least 25% per 1,000 liters of milk. In practice, this translates to "not much" when using for small batches. And by not much, I mean something like 1/64 tsp per gallon.
2. How much of a culture do you buy at once and how many batches is an amount good for?
Again, which culture? p roqueforti? Well, that's up to you. How many batches are you going to do before the culture degrades to the point of not being usable? If you are making 1,000 liters every day, you buy enough to last you however long before the culture expires. I guess I'm not really understanding your question. You buy the amount necessary to use it up before it goes bad based on your batch size and make frequency.
3. I've seen the term starter used. Is this a way of propagating the culture create more to use? How does this work exactly?
Yes, please search the forum for the word propagate and propagation. I, Sailor, and others have covered this in multiple threads.