SEO is a fast moving area that the rules changes without notice. I've seen single person companies try this but since it's a big topic and field your best results are with a team of folk with a wide plan to cover all the angles. That is you usually have a leader with experts in each sub field.
I know that sounds horrible but it's become a minefield where I've yet to find one person that knows it well.
What we need minimum when starting up a SEO Firm. How many resources we need to handle medium to large scale vendors about 10 clients per month. A single person can able to do it well.

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