Every Prerelease there is a lot of tension between LGS owners and the playerbase about exactly how much prize support is available.
There are several circulating claims (and at least one mothership article implying) that Wizards provides 2 packs in prize support per player, and expects those prizes to be distributed at the prerelease. Yet every year, there are horror stories about stores splitting just one box among a 40 player field, etc. A lot of tempers get raised, and people throw around threats to tell Wizards.
It would be good to have a definitive answer about whether there are in fact prize minimums, or it is all ultimately up to the store owner's discretion.
I have talked about this with both of the LGS owners in my town, and even among them there seems to be a lot of uncertainty on this issue.
The relevant text is as follows: We recommend that at least 2 Born of the Gods packs be added to the prize pool for each player in the event. You may give out additional Born of the Gods packs as prizes while your supplies last.
I don't 100% remember if the instructions were the same for past kits, but I know there has been similar (if not identical) wording going back to at least Return to Ravnica when I started doing TO work.
It used to be mandatory I believe. Now, you get materials for 20 players and a booster box for prizes. IE. If you use all 20 prerelease packages you will be a bit short on prizes but not much.