Starting with low-numbered routes:
1: US, CA (2, those seem to be rare)
2: US (west), US (east), CA, MA, OH, RI, NY, WV (8)
3: US, CA, ME, MO, VA, WA (6, not counting MA-3, which is an extension of US-3)
4: US, I, CA, IL, MA, ME, WA (7)
5: US, I, CO, NY, PA, VA (6)
6: US, OR (2)
7: US, CA (2)
8: I, MA, NY, PA, UT (5)
9: US, CA, CO, DE, MA, ME, NH, UT, VT (9, not counting several NY alternates)
10: I, DE, MA, NH, OR, PA, WA (7)
11: US, I, CA, NH, NY, VT (6)
12: US, I, CA, MA, NH, PA, UT (6)
95: US, I, AZ (unrelated to US-95), NV (4)
99: BC, CA, OR, WA (formerly contiguous, but now separate), MA (5)
128: CA, CO, MA, MD, UT (5)
Well, that's an hour I'll never get back...