It's hard to say really. Our community is pretty small as it is, or at least from my point of view based on who shows up to my monthly events. Last event we had 15+ show I think? So as a conservative/ underestimating guess: 20+ people for Brawl. At least 20 because I know that is how many Brawl players I know LA can supply between the competitive and casual players, noob and verteran. The rest will be whoever from out of state bothered to pay attention to us and take us seriously enough to make it out to this. As long as we provide a good service this convention and show those who attend/watch the event, that we mean business, our rep will go up and we will see growing numbers of entrants for future conventions. It's all about the rep building right now. :D
It also helps if everyone in the community shows long term hype for these events, not just the TO's. After all, why should OOS players come to an event the locals aren't even hyped about?