Active Directory increments the USN each time a change is made in AD until it reaches the maximum USN. As the USB is stored as a 64 bit number made of 8 bytes, the maximum USN in AD is:
That’s 2 to the power of 64 minus 1. This means that if you had 100,000,000 changes every second in Active Directory, your USN would reach the maximum number in year 31,241.
Moral of the story: You don’t need to worry about the maximum USN, Active Directory isn’t going to run out of numbers any time soon.