WordPress database scalability from the code perspective

If INT is set to UNSIGNED, allowing only non-negative integers, your value range is from 0 to 4294967295.

Thats…

4,294,967,295 (4 billion +).

…for the given table. Just to put things into perspective.

Only through poor management and unnecessary incrementation would you exhaust that range, say for your posts table as an example.

That said if you have some unique requirement that uses that entire range for a table then simply create another. WordPress allows for that.

There’s also the possibility for multiple DBs (http://codex.wordpress.org/HyperDB) being one example.

If you’re pushing the upper limits of a standard table primary key, then you would already be scaling your application beyond the traditional single table schema to handle such quantities of content.

Notice how I said application and not blog? At this point, its likely that your site resembles nothing like the common blog does and instead is much more an application, a data management system.