Quick Question: How few search results must there be to rank competitively for a long-tail keyword? Of course the lower the better. But is there a point, say 500k search results, where it is still possible to competitively rank for a keyword?
Search results is that result which is we find in site search field. Search engine ranking is that which is we find on search engines. While we search our keyword in search engine ranking.
This depends on a lot of factors. You should be looking beyond search results (sure you can use the number but look beyond just this) and into the top ranking pages/sites. How old are they, how many back links per page do they have (and how many of those are high PR), how many exact-phrase mentions are there on the web, how many in titles and URLs, adwords competition.. From what I have seen though, under 500k will usually be fairly easy to get to the top, it does vary from industry to industry though so further research is recommended.