Especially since (as it says at the top of this page:
http://asterisk.hosting.lv ) you may have to pay a royalty to use the g.729 algorhythms. Digium makes it easy ($10 per channel) to pay and track your payments. But be careful, if your system crashes they will only issue you a "refresher" license once. So make sure you are WELL backed up after you install your licenses.
Licenses are for concurrent use ... you don't have to buy one for every channel you use, just for enough to cover the LIVE channels you will be using at the same time with g729.
So if you have 30 channels with your provider, but you only intend to use 15 with g729 at the MOST at any given moment in time (presumably leaving the rest with gsm or ulaw), you only need 15. But your 16th g729 call WILL fail.
And yes, you can start with 10 and add a few every week until you're full (adding agents = adding licenses).