I am considering buying an Extreme Vape but I was wondering what is the minimal amount of herb needed to run it.
Any amount will do, but the smaller the load the thinner the vapor will be. The Extreme has a pretty large bowl and airways, so a small load won't provide much visible vapor.

I have had friends that said the Vapolution is the best for small amounts.
I've been using a Vapo for a couple of years. I'd agree that it used to be the best for small amounts, but the new Purple-Days vape now has it beat for efficiency. It's tops for efficiency and using small, measured loads. It also never burns your herb, which can be a problem with the Vapolution.