One of the most common questions prospective clients ask SEO (Search Engine Optimization) freelancers/agencies/consultants is something to the effect of, "can you get my site ranking number one?" Can a site be ranked number one via SEO? Sure! Can that be guaranteed? No. There are a number of factors that come into play, such as how competitive the keywords are that you're looking to rank for, the quantity and/or quality of links pointing to your site vs. the competition you have to outrank, and much, much more. In the end, though, a rankings just can't be guaranteed (and they shouldn't be -- even if the SEO you're looking to hire does guarantee it and possesses the skills to get the job done in a completely ethical manner).
With that said, below is a video by Matt Cutts (from Google) from back in October which offers an explanation to SEOs that enables them to show prospective clients why they can't guarantee number one rankings. Likewise, Matt delves a bit into what clients should really focus on instead of fixating on the desire to rank for what they think are the most adequate and relevant keywords to their business. I've also covered the topic of keyword focus in a previous post titled "The One Mistake Many CEOs Make About SEO" which you may be interested in checking out after watching the short video below:
For reference, here is the page Matt Cutts references where Google states its stance on placement guarantees and below is the exact quote:
No one can guarantee a #1 ranking on Google.
Beware of SEOs that claim to guarantee rankings, allege a "special relationship" with Google, or advertise a "priority submit" to Google. There is no priority submit for Google. In fact, the only way to submit a site to Google directly is through our Add URL page or by submitting a Sitemap and you can do this yourself at no cost whatsoever.
So, there you have it! A simple explanation straight from Google as to why you shouldn't expect/accept (if you're a prospective client) or offer (if you're an SEO freelancer/agency) a guarantee for number one rankings.