I was an only bridesmaid in a wedding a few years ago and there were 2 groomsmen. It worked okay - the photographer had to be a bit more creative I think, but I love the photos from that wedding and think she did a fabulous job making them look even and balanced. As far as things like walking back up the aisle, I had a groomsmen on either side of me (thankfully it was a wide aisle). We all danced with our own partners for the bridal waltz rather than just the bridal party.