I wanted to do this for my game as well, so I went digging in the files myself.
Currently for .701 and .702 the way to change the number of arrows / bolts / whatever in a bundle has changed slightly.
Like knight_9 said, go to "C:\Program Files\Mount&Blade\Modules\Native" and look for the "item_kinds1.txt" file. You should definitely make a copy of it to back it up in case you screw something up. Then open the file and look for the lines which contain "arrow", "bolt", or the name of the throwing weapon you want to change (such as "javelin" or "throwing knife").
On that line, look towards the end of it for the third to last number. For "practice arrows" (the ones in the arena, I assume) the number is 80, for any other arrows the number is 24, for javelins it is 7, etc. This is the default number of items in the bundle, so all you need to do is change it to a higher number. The same goes for bolts and throwing weapons, the third to last number on it's line.
So, if you change the number to say 48 for all arrows, then all regular bundles of arrows will have 48 in the game, while "large" bundles in the game will have 50-something.
I'm glad the file format has been changed to make things easier to edit, even if the editor program isn't working for the current version.
EDIT: This is for 0.701 and 0.702. Be sure you're in the "Modules" folder, NOT the "Mods" folder. Apparently the "Mods" folder is a leftover of older versions of the game.