Soda Raft Question

And if any soda company would give me money, I'd use their
brand name in the problem statement. :-)

The following is a true story.  I use it as a problem in static fluids every year.  This year I assigned it when I was absent -- an upcoming post explains how I assigned the problem in class.  

For now, though, look at the poll at the left of the blog -- vote for your estimate!

1. Mr. Jacobs’ friend Brian Jackson saved two-liter soda bottles throughout his senior year of college.  During “Haverfest," he duct taped the bottles together to form a raft.  He then successfully floated himself out onto the duck pond.

Estimate how many bottles Brian used.  Explain your reasoning thoroughly and show all calculations for full credit.