I’m sure there used to be a setting for how many posts it would show in total before there was a page break, but now it seems to be missing? I remember setting it to 12, with the first three being “full posts before grid”, but I can’t find it.
It didn’t matter so much when the auto-loading of posts worked, because you could just scroll down to load more, but now that it ISN’T working (see support thread Image questions, but basically: set “full posts before grid” to zero and auto-load breaks) … it’s a bit frustrating, because it’s only loading nine posts at a time and I can’t change it.
The Posts per Page option is WordPress-specific and can be found under Settings > Reading.