Same here, only about 2-3 cultures at a time so I mix cup by cup. I think that mixing a large amount would be harder to evenly divide into each culture, never tried it so can't say for sure. Doing it cup by cup is really easy though.
If I'm using my homemade mix, I mix it in a big batch so I can get some of the additives evenly distributed and then portion it out.. It isn't that hard, set the cultures up in a row and then go down the row adding about the same amount to the culture with a spoon (or as Mr. Brown on Good Eats does use a disher or ice cream scoop) and then repeat. I use a spatula to remove the last in the bowl and add it.. it takes me about the same amount of time as to individually portion and mix the Superfly in individual cultures.