I would figure it should carry 24, to have 3 shots per tube.
Azile, I could write up a very very long and detailed explanation about how to get these into the game, but it would probably be easier to wait until GalEmp releases them all as one big pack in a few days. In the meantime, all I can suggest is looking at the wiki. If you're trying to put them into FS2, you'll want to create a mod folder for them. In that folder, you would of course want a data folder, and in that you would want your models, tables, maps, effects, and sounds folders. You will want to take the sounds.tbl file from retail FS2 and copy it to the sounds folder. Then either do the same with the weapons.tbl, or create a .tbm file, either one for each weapon or one for all the new weapons. Add the info from the table.txt files into the new tbl and tbm files. Make sure when adding to the sounds.tbl file that you overwrite the correct indexes. You will also want to add the 'in tech database' flag, or whatever it is, to each weapon's flags. That way you can see if they're working or not by going to the tech room. _Then_, you will need to edit or create a new mission in order to actually make use of these weapons. These aren't being released in a manner intended for anyone to be able to just start messing with them, if you don't already have some modding experience it's probably not even worth it, but if you really want to use them, it'll take some time to figure out all the stuff necessary. It may help to give a better description of what you've tried so someone might be able to guide you in the right direction.