An interesting mathematical question.
Shadowdale's calculations are correct except for one variable: for some reason, "all happy" only applies to those "working the land"; i.e., only the first 20 citizens in any city can qualify as happy (for the x2 bonus).
As for unreachable ... it would take at least 42,000 food caravans/freight to max out every city to 127 ({127-c.42} x254 x2) ... when I sober up, I think I can actually mathematically prove that there just aren't enough turns in the game to produce 42,000+ food caravans.
(And sometimes, I really hate mathematics!!!)