For Ranger School you are required to bring twelve razors.
For Sapper Leader Course you need to bring a 30 day supply.
For SFAS you are required to bring six razors.
I recommend you bring cheap disposable ones with no more than two blades. The fancy five blade razors don't work well when you're shaving out of your canteen cup everyday.
These twelve individually wrapped twin blade razors are disposable and come inside a 6 x 9" 6 Mil Zip-Lock Plastic Bag, which perfect for items you don't want to get wet.