I'm basically going to use my Travel Mapping data as the source for this.
As far as the so-called "Multi-State Routes" at the state level, I will count each state as a separate entry. I don't consider there to be such a concept as "Multi-State Routes".
Interstate and US highways will be counted once, regardless of the number of states entered.
Discontiguous segments of what would otherwise be the same highway listed once.
My top three:
1: (13, counting A1A as a "1")
US 1, US1 Bus (Bel Air, MD), US1 Byp (Portsmouth, NH), US1 Alt (York, ME), US1 Alt (Washington, DC), US1 Alt (Baltimore, MD) US1 Alt (Bangor, ME)
FL A1A, CA 1, DE 1, DE 1D, GA 1,
TH 1 (Iceland "Ring Road")
11: (10)
I-11, US 11, US 11E, US 11W
MI 11, NH 11, NC 11, OH 11, KY 11, ME 11
295: (10)
I-295 (NC), I-295 (RI/MA), I-295 (ME), I-295 (FL), I-295 (VA), I-295 (MD/DC), I-295 (DE/NJ/PA),
DC 295, MD 295, FL 295 (Pensacola)
And since others have been doing this:
997 is my highest number for which I have two routes with the same number. FL 997 and PA 997 (I have also been on MD 997D, 997E, and 997F, which would make five, but none of those are signed)
896 is my highest number for which I have three routes with the same number. DE 896, MD 896, PA 896 (But I would understand if you consider this sketchy)
895 is my highest number for which I have four routes with the same number. I-895 (Maryland) I-895 Spur (Ferndale, MD) I-895 Spur (Furnace Branch, MD) VA 895 (same note about being sketchy applies)
Unambiguously,
581 is my highest number for which I have three routes with the same number. I-581, NC 581, PA 581
495 is my highest number for which I have four routes with the same number. I-495 (MD/DC/VA), I-495 (DE), I-495 (MA), I-495 (ME)