Antenna's don't "handle traffic". The antenna shapes the signal, and therefore affects the range. Go back and read what Starpoint said. Four hundred feet is possible under some circumstances, but if you're asking questions like "can antenna can handle traffic?" it sounds like you're probably in over you're head. Suggestion: get some help to properly do your signal engineering.
The AP is what handles the traffic. Most SOHO will handle 32 users, but some will only handle 8, and some will handle 256.
Assuming that the AP will handle the number of users you anticipate, then you have to consider the traffic throughput. Throughput will be affected by the number of users on at one time, and what traffic each user is generating. 256 users, all working on Word documents and checking email occasionally, won't slow down anyone else. However, a single user downloading an HD movie might slow everyone else to a crawl if you don't configure the traffic shaping correctly.