[Source:] Comparative subordination
Examples of the comparative that do not allow an analysis in terms of coordination (because the necessary parallel structures are not present) are instances of comparative subordination. In such cases, than has the status of a preposition or a subordinator (subordinate conjunction), e.g.
c. More passengers than the airline had issued tickets [♦ to ♦ ] tried to board the plane.
d. More guests than we had chairs [♦ for ♦] showed up.
Please beware that I was the one who added the prepositions surrounded by the lozenges (♦ ). I did so because I believe that these were wrongfully missing. Were there? Why or why not?