I might be wrong here but "Founder" is just a title and it has nothing to do with law or investors. Adding another "Director"/"Officer" requires some paperwork though.
It varies in incorporation requirements by state, but most only require 2 titles positions - a President and a Treasurer (the “Officers”)
Adding or removing a Director, as in “Board of Directors”, requires amendment to the Corporation’s bylaws, which should state how many seats there are.
Aside from those titles/roles, all the “C” prefixed titles are made up. Usually, one C level person is the President, another the treasurer. Co-founders are usually on the board of directors. Many times, a single person can be on the board, a co-founder, and ceo.