Neptunians count in base 8, with number symbols that also show a sub-base of 4. Like many number systems, this is based on hand-counting. Standard Neptunian morphology does include 10 fingers, but those fingers are webbed, and the webbing is actually used in counting, rather than the fingers. Webbing makes the extension of single fingers difficult, so numbers are used by spreading fingers to show the webs between, which are often more brightly-colored and visible than the digits themselves, anyway.
(Neptunian number symbols go here)