Seven harvests, gathered with deliberate care. Targum Pseudo-Jonathan on Genesis 41:48 records the logistics Joseph used: "he laid up the produce in the cities; the produce of the fields which were round about a city he laid up therein."
The principle of local storage
The detail reads like a throwaway, but it is a policy. Grain grown near city A was stored in city A. Grain from the fields of city B stayed in city B. The Aramaic paraphrase, which reached its final form in the Land of Israel around the seventh or eighth century CE, preserves this because the rabbinic tradition saw its practical wisdom. Bereishit Rabbah 90:5, a rabbinic commentary on Genesis compiled in the Land of Israel around the fifth century CE, offers a complementary reason: grain keeps better among its own soil and climate, because the land's own air preserves it. Modern agronomy would call it micro-climate acclimation.
Why distribution outperforms centralization
Joseph's system meant that when the famine arrived, every Egyptian city could open its own reserves without waiting for a caravan from a distant capital. Transportation would collapse during the famine years anyway — hungry animals cannot pull wagons far. Local storage was the only storage that would still be reachable when things got bad.
The takeaway
Joseph's second instinct, after collecting the grain, was not to hoard it centrally but to keep it close to the people who would need it. The best plan is one that still works when transportation fails.