* Service based companies can often rely on a larger greenbelt population to meet their needs.
* Complexity should be considered. If a company has extremely complex products or processes, a larger percentage of black belts may be needed.
* Additionally having full-time belts or part-time belts would need to be considered.
* Is the site able to support a larger amount of change that a large population of belts may drive?-
Personally having approximately 1% belts has worked well.
In lieu of importing new FTEs or making your staff study for and write 4 hour exams, a contingency is to build up a Six Sigma Knowledge Base.
The Knowledge Base (being a Sharepoint, Wiki Site, or equivalent) can be a point of reference for employees and team members to get templates, references, and examples.
I absolutely agree that the complexity and scope of project work play a huge part, but something to also take into consideration is where you currently are in the journey of culture and practice. Depending on where you are on that journey, you may require more or less of the belts present. Nonetheless, a core group (size depending) will always be valuable.