No, there is no way to set a number that will *always* be displayed, due to the way that YARPP works. The philosophy is that, if your site actually doesn't have anything related, don't artificially show anything as related.
For example: you always write about food, and then once talk about your elevators. That elevator post shouldn't have anything show up as related, unless they really are. That's the idea.
In general if you think YARPP is ignoring things which *are* actually relevant, though, try lowering the match threshold to make the algorithm more lenient:
In the YARPP options page, if you don't see a "Relatedness" section, go to "screen options" in the top corner to show the Relatedness section. Then lower the "match threshold" (default 5, maybe to 2?) and save. Then see if the recomputed results look better to you.