I'm a Lisp programmer, damn it! I will use one of the fingers for one tag bit which indicates a pointer into the heap when zero, and from there I will boostrap bignums somehow, and the rest of the numeric tower.

You can count from zero to two on two fingers, so two-fingered aliens may invent numerals for zero, one and two. If they develop that into a place value system, it will be (fingers+1)-ary, or in their case trinary.