Yeah... that's one of the things you can change while mIRC is running. The difficulty with the favorites one is that you can't just use your own number system (such as starting adding your channels at n1000) in order to prevent having to check what number to use... you can't have a break in the numbers or it won't work.
Of course, if you're making your own dialog with channels, you're already loading the list with a loop and will know how many there are. If all you're doing is making a right click option, then you still have to run a loop to make it work correctly (I think). As far as I know, there's no way to do something like $readini(mirc.ini,chanfolders,0) to get the number of items in that topic.... or is there?