No one can guarantee a number one ranking. Some companies offer various "guarantees" but they often are vague in some areas or they offer the company an opportunity to improve rankings and if they fail the client receives their money back.
Here's the challenge for you. Many good SEOs have a full client load. They have an established history and can charge based on their time. There is no need to engage is any arrangements where profits are split or other types of pay-for-results type of contracts.
If you wish to pay for a ranking, then the pay would be substantially higher then normal SEO rates. The normal format of such an arrangement would be you pay the agreed upon rate for the work, then offer a bonus for achieving the #1 ranking.
With the above understood, it is rarely a good deal for the client to engage is such arrangements. Your interest is PROFIT, or at the very least conversions, not ranking. Some issues which may occur: you rank #1 for a low traffic keyword, you rank well for a keyword which does not convert well, your listing is not optimized for click through rate, etc.
SEO is a comprehensive process which involves all aspects of your site and business. The goal of SEO is likely to increase your company's profitability. That should be the focus of a good SEO campaign. If you are targeting specific metrics like ranking, PR, CTR, conversion rate, traffic, etc. you are likely missing out on major factors which influence your profitability.