+1 on four straps. I know people do it with less, but if your one strap (or one of the two) come loose and it starts moving around, things get damaged really fast. Ratchet straps and a few minutes of your time are cheaper than repairs.
If you use four straps, and one breaks or loosens up, it's usually not the end of the world.
2015 XLR Hyperlite 30HFS5 (mods being performed regularly)
2009 Salem LA 292fkds (gone)
Nights- ('12)23 ('13)23 ('14)15 ('15)31 ('16)25
2014 Harley Davidson FLHX (XLR cargo)
2014 VW Jetta TDI (diesel sipper, camping support vehicle)
2011 Ram 2500 CC 4X4 CTD, B&W Companion (toy hauler hauler)