Student or Learner
The problem is that we have eight fewer/less stamps than there are letters that need to be mailed.
Which word in bold should I use?
To help you decide in the future, if you can count them -- and you can here: one stamp, two stamps, three stamps -- use "fewer."
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.