"(20) 2-gallon batches out of a 50-DCU packet" not sure if I understand this part
Achik, part of what is confusing is the difference between using cultures in a commercial setting vs. using them in a home/hobby setting. In a commercial setting, they are making cheese 600 litres at a time, so they would be looking for a 50 DCU packet to put into that 600-litre vat. At home, you are going to make, say, 10-16 litre batches. In theory, with very, very precise scales, you could measure the weight of the contents of the packet and divide it up according to the ratio of 1 packet per 600 litres of milk. In practice, though, most of us find it easier just to use small measuring spoons (ones that can measure not just 5, 2.5., 1.25 ml, but also .625, .3125, .01625 ml -- or in other words, not just 1 tsp, 1/2 tsp, and 1/4 tsp, but also 1/8 tsp, 1/16 tsp, and 1/32 tsp). Many recipes call for "1/8 tsp TA061" or something like that. In reality, since the strength of one batch of culture varies from the next, 1/8 tsp might be more or less innoculation than is really needed -- and the recipes err on the side of more. Thus, even though the packet might provide up to 600 liters, in home use it might only last for, say 200 liters, because, following the recipe, you are using more each time than is really necessary.
This does not hurt anything, as long as you keep an eye on the process, especially the pH, to make sure you are not over-acidifying and getting something other than what you are aiming for. With experience, you get to know how "strong" a given packet it, and you can make adjustments accordingly.