I host an event that usually draws 200 people. It is held in a university lecture hall that holds about 260 people. How many/what kind of wifi access points/routers will I likely need to provide everyone reliable access?
First, if you're not using the universities existing wifi you'll certainly want to get it turned off in that area to reduce interference.
Second, two or three decent AP's (channels 1, 6 and 11) should be more then enough, and if relevant, give the presenter a wired connection just in case.
At this point use 802.11 G AP's (or a/g), N ap's aren't really there yet. Cisco 1200's can be had on eBay fairly cheaply these days and are built like tanks.
Your answer completely depends on the amount of traffic you expect. The number of people you have attending is a function, but you'll also want to think about the average usage per person, your possible peak traffic, and coverage area.
Essentially, plan for the worst, and then use your estimates to figure out what you don't need.
If your coverage area is open, and indoors, that reduces the number of APs needed. If you don't expect people will be streaming video, that reduces the number of APs. If you don't believe there will be a moment of peak traffic, that reduces APs.