Things have changed a lot in the last two decades as most rural counties have gone to new “city style” addressing systems that number from a baseline. The result is many, many house numbers in the high five digits. In eastern San Bernardino County, California (nation’s largest county) I took a picture of 153243 Cahto Rd., Earp, California. Looking at some maps, I suspect that San Bernardino County must have house numbers even higher than that.
Complicating this question are the unusual addressing systems used near Chicago in DuPage and Kane counties, which count number of miles from State and Madison in the Loop and produce results such as 46W330 Burlington Rd. Milepost addressing schemes on Florida’s Overseas Highway and in rural parts of a couple of California counties produce numbers as high as 120162 (literal meaning 120.162 miles from baseline).
Some Canadian counties have numbering schemes that produce six-digit house numbers, but the first three digits indicate an area and road number rather than distance from a baseline. O‘ahu uses a similar system.