What chapter are you referring to? Is this in Yelp design?

I believe he's referring to this part in Section 4. Database Schema:

Although a four bytes number can uniquely identify 500M locations, with future growth in mind, we will go with 8 bytes for LocationID.

We don't need base 64 encoding. A four byte number can give us 2^32 unique IDs.

Assuming "500K" in the question was a typo (should have been "500M" per the course material), 4 bytes * 8 bits is 32, and 2^32 yields 4,294,967,296 potential values.

That's correct.