How might diet influence the number of humans that earth can ultimately support?

A diet high in meat takes a larger surface area to support.
Hypothetically there could be more people if people ate at a lower trophic level, i.e. were mostly vegetarian.

Edit:
See your other question!
https://answers.yahoo.com/question/index?qid=20150512093222AA4m5ES

Number Of Humans On Earth