How to promote people in a small company of less than 20 people?

We are a small company of less than 20 people. How do we provide opportunities for people to move up the ladder? We are not growing in terms of company size as our business runs pretty well at it's current size. What type of corporate hierarchy would you recommend for a startup this size?

Difficult to say. Assuming it's mostly an engineering team, you want to limit how many middle managers you have in such a small organization.

People are happier when they work on more important stuff and get to make their own decisions. It doesn't mean you should promote most senior people to managers, but trusting them would go a long way to keep them happy. Some engineers will want to go deep into technical issues, others like to be exposed to the business side of things. Look up job titles such as "product managers" (who don't manage anyone) and "tech lead", for ideas.