Five and a half....
425 sq feet does not sound like a very large building, thats just a bit bigger than a 20x20 area; 1 AP should be able to handle a 20x20 space with ease.
No seriously - you should do a site survey if possible. If the building is not built yet, perhaps try doing a survey in a similar size building. Keep in mind that there are several factors that come into play such as the structure itself, coverage areas, capacity, throughput, interference, cubicles, machinery, other frequencies, etc.
If the building has multiple floors, you will have to plan around elevator shafts, stairwells, radio overlap from one floor to the next.
Also plan for the wired infrastructure to support the AP deployment.
Do you know which freq you will be using with N-draft? N-draft is capable of channel bonding
where it "bonds" two channels together for higher bitrate, coverage,etc. This is great in the 5GHz freq, but limits your choices in the 2.4 GHz freq as there are only 3 non-overlapping channels. here is some good info on 802.11n draft:
http://www.cisco.com/en/US/netsol/ns767/netbr0900aecd806b8a92.html
