Not alone, but as a team I bet he did. Just as Fritz Haber did not personally save countless millions with his discovery, thousands of other people had to be involved there as well.
Billions. The total carrying capacity of the Earth maxes out at 4bn people, even with maximally optimized agriculture and a vegetarian diet. The extra half of the Earth's population today owe their existence to Haber-Bosch.
It really was a two-man team that discovered nitrogen fixation - the other being not Carl Bosch, but Robert Le Rossignol who assisted Haber in developing his bench-scale nitrogen fixing reactor. Carl Bosch led the team that scaled it up, and that was a large effort of hundreds of people.
Please not that this was in relation to whether or not a general doctor saves lives… A lot of work is indirect.
The Haber–Bosch process was never a two‑man mission to “feed the world.” Haber devised a laboratory method to fix nitrogen, and Bosch led the first industrial‑scale implementation, but neither of them was personally producing, optimizing, transporting, and applying fertilizer across the globe. Turning that reaction into most of the world’s nitrogen fertilizer required teams of chemists, engineers, metalworkers, factory operators, agronomists, logistics workers, and farmers, and an entire century of industrial expansion. If it truly were a two‑person accomplishment, we would only need a handful of farmers per country, which is obviously not how agriculture or industry work.
There is no reason to believe that a lack of nitrogen was a problem in particular. It seems that most effort was spent on getting fertilizers with phosphorus and other minerals, nitrogen was secondary, as many plants can obtain it from the air. If anything, it allows our modern, heavily cereal skewed diet. Poor nutrition rarely meant an absolute lack of food, most of the time it only meant insufficuent quality, and the green revolution was a massive step backward in that regard
That is literally true, but for anyone who hasn't studied plant biology, I think that "some plants have evolved specific structures to host obligate symbiotic bacteria that obtain nitrogen from the air and convert it into a form usable by the plant" is close enough to "many plants can obtain [nitrogen] from the air".
