What if I don't know how much traffic we get?

No worries - just pick whatever plan seems closest to your best guess, and ethnio will tell you what your actual pageviews are once you start using it. One handy thing about Javascript is that it makes it very easy to calculate page views.  Just go to you account  >> Plan & Usage and you'll see it at the bottom of the page.  

If you exceed whatever plan you selected for your account, ethnio will let you know when you need to make changes.