I think there's an expert opinion in regards to a numerical quantity, but Google defines a searchable page as something that receives a lot of inbound links (and you can make the landing page have a lot of long-tail keywords that don't duplicate the original keyword as much).
If it starts reading like a content farm, I'd re-think the purpose and aim of the page.
Google also ranks based on natural language, so if it reads quirky to you, it also might cost you some search bucks. Try to write as plainly as possible, and keep the page theme-oriented. Making the pages sticky, content-fresh and keyword rich would be my aim.
Going out on a limb and throwing a number out: I'd say you can go as high as 30-40 if you have long-tail words, you use the words in different but spatially similar context, and definitely not duplicate any content from other landing pages.
Try some multi-variate testing with different saturation levels 
kevin
