Percentage of Ownership
I presume you are referring to percentage of ownership of an existing corporation.
Technically, you can learn about the total number of shares of a corporation from its Articles of Incorporation. However specific percentage of ownership of each shareholder is listed in bylaws of the corporation, which in case of private corporations is not a public document. You need to be an authorized person to have access to this document.
For LLCs the percentage of membership interest is defined by the members in the Operating Agreement, which is also an internal company document.