What would be the number for what is Exit 880 on I-10? And would that be the highest numbered sequential exit on our interstate system?
I-5 in California could be a rival. It's a bit shorter than I-10 in Texas, but runs through 3 large metropolitan areas with boatloads of exits.
Using the tables on Wikipedia, my quick estimate is 368.
By my quick estimate, California wins, with I-5 having about 420. The spacing made those harder to count, so that's an approximate number, but I'd say the margin is large enough for reasonable certainty that I-5 has more.
I counted 395 on I-35 in Texas on Wikipedia and included 35E since it's part of the mainline. Kinda crazy considering it's about 375 miles shorter than 10, but it hits quite a bit of metro as well.