I can't find the number "120" in the source paper

I don't actually predict 120 by 2050. I am more interested in how our life history should change given radical life extension, whenever it happens to occur. I state this at the end of the paper:

This analysis should prove useful for the impor- tant reason that you can make a fairly reliable prediction for biological reproduction given radical life extension (RLE). If RLE is not achieved before 2050, but instead at some later date, 100, 150, or 500 years from now, this will likely coincide with the indefinite postponement of current biological reproduction in favour of current cultural repro- duction. Therefore, any popular or political opposition to the practical application of RLE breakthroughs on the basis that they would lead to catastrophic overpopulation issues, are almost definitely unfounded. Also, any scientists cur- rently involved in research related to RLE should not fear that their breakthroughs will lead to major population prob- lems that will need to be solved at some future period of time.