No SEO Agency can guarantee rankings, regardless of what their claims may be. There are changes being made to Google's algorithm on a daily basis along with a variety of technical issues that need to be addressed on a website, as well as competitors in your niche. These variety of factors make it difficult to fulfill any concrete guarantees. What we can do, however, is create a comprehensive audit and build a development plan that will help increase your organic visibility over time.
KPI’s can also be set to have a goal to work towards and achieve. However, any KPI’s need to be set after a comprehensive analysis of a website's current traffic and rankings.