I agree it would be impossible to make a clear ratio. However it should be possible to look up every mention of a married woman, and divide into:
- Certain / probable polygamy (more than one wife mentioned, probably overlapping: Moses, David...)
- Unknown (one wife mentioned with little context, two wives mentioned but likely to not overlap: Boaz, Ahab...)
- Probable monogamy (one wife mentioned in circumstances that make monogamy probable: Adam, Noah...)
I expect such a breakdown could be a real eye-opener for people if they were willing to look at it. I expect the polygamy numbers would be comparable to the unknowns, and the probably monogamous could be counted on one hand. Such a breakdown would allow you to demonstrate a minimum polygamy ratio presuming all unknowns were monogamous, and a maximum ratio if they were all polygamous, which would be interesting.
Actually, if you wanted to get really complicated it would be possible to apply Bayesian statistics to the issue, assign a probability to each marriage and obtain a probable polygamy / monogamy ratio with a confidence interval - but I highly doubt that would persuade an ardent monogamist anyway!