I am sure I have read this, but not able to find it right now (a little under the weather). I know the definition of a structure is a walled and roofed building. Question is, how many walls are required? I believe 4, but not 100% right now. Thanks in advance!

From the FEMA Flood Insurance Manual's Definitions Section (available here):

Building.
• A structure with 2 or more outside rigid walls and
a fully secured roof, that is affixed to a permanent
site; or
• A manufactured home (a “manufactured home,”
also known as a mobile home, is a structure built
on a permanent chassis, transported to its site in
1 or more sections, and affixed to a permanent
foundation); or
• A travel trailer without wheels, built on a chassis and
affixed to a permanent foundation, that is regulated
under the community’s floodplain management and
building ordinances or laws.
“Building” does not mean a gas or liquid storage tank
or a recreational vehicle, a park trailer, or other similar
vehicle, except as described above.

GuitarDude is right, but the regulatory definition doesn't say 2 walls. What about a grain bin? It really only has 1 wall.