OK. Since you're dealing with polymers produced on this planet by typically understood chemistry, you have an average, which covers the range of possible polymer lengths. What you seem to want, an exact number, can't be found.
Now you can take a sample, try to figure out how much is there analytically, and use that number, like Borek: suggested.
Or you can just add one reactant in vast excess, and wash it away by some means, as is often done in production chemistry.