FYI, typically if you are a bridesmaid, you buy your dress. The bride just happens to pick it out. Also, only about half of the weddings I've been to have had all the bridesmaids wearing the exact same dress. They'll always be the same color and usually same length, but the actual dress will be different. (Like one girl will have a strapless while another has a halter and the last has spaghetti straps or whatever.) It's really up to the people getting married. For instance, one of my best friends isn't going to have a maid of honor, her brother is going to be her "dude of honor." When my cousin got married earlier this summer, he and his wife didn't have any bridesmaids or groomsmen. (They also had a good friend get ordained so he could do the vows and that was pretty awesome.) The only reason I can think of having the same number of people on each side is aesthetics - it looks better if you're flanked on both sides by the same number of people.
Personally, I want to make my wedding as not boring as possible and watching lots of bridesmaids walking down an aisle is kind of boring! One to hold your bouquet and maybe one or two more to catch you if you pass out sounds reasonable to me. Maybe they're traditionally there to keep you from running away, who knows.