According to the manufacturer’s website, they sell four different 4 count packs of various color combos. In addition to that, their site also lists packs each of 6 and 12 counts (those seem to be available in multiple configurations as well). There are also packs of 24, 36, 48, 60, and 90, but everything I’ve seen suggests that there aren’t different configurations for the 24 count and larger packs.
I wanted all the colors, so when I put them on my Christmas list in 2017, I asked for and received the 80 pack. That doesn’t seem to be manufactured any longer (though some places still have the 80 in stock) and has been replaced with the new 90 pack that contains 10 new, additional colors.
Across multiple sites, I only see reference to one version of 24 color pack with the exception of one that comes with a different type of Kuretake brand pen as well, but that may not be accurate. If you’re wanting to expand your collection beyond the 24 pack, you may have to wind up with duplicates from the 24 by buying one of the larger packs. Though they are available open stock, I’d imagine it’s not cost effective to purchase them that way unless you’re only wanting a few individual colors. I plan to purchase a 12 pack that’s available that contains the new 10 colors plus 2 in black. I hope this helps!