The number of pages that qualify for inclusion in a SERP has some bearing on how difficult it will be to earn top rankings. However a more important factor is how much money is changing hands. There are some search terms with few sites competing but fierce competition.
IMO, better measures of difficulty are the amount of bidding on ppc ads and the number of backlinks hitting the pages with top rankings.
* "It's not the size of the dog in the fight that matters, it's the size of the fight in the dog." Mark Twain
* "Free advice isn't worth much. Cheap advice is worth even less." EGOL