Where is this setting?

Am I missing it?

Thanks

Just input "posts per page" in the ACP search, and there it is.

Note that this also sets the number of topics shown on a forum page.

In IPS 3 it was possible to have, for example, 25 posts per topic page, and 40 topics per forum page.

That distinction is no longer possible in IPS 4, which I think is a regression, but that's how it is.