Company only with manager/owner

ISO 45001 is applicable for any organization in any industry, so it can definitely be used by a company with a single owner/manager. The requirements are written to be descriptive but not prescriptive, meaning they tell you what needs to be done but not how to do it. So, you can perform the necessary processes with the employees you have, and in the best way for your organization.
For more information on what is included in the requirements, see our whitepaper: Clause-by-clause explanation of ISO 45001:2018, https://info.advisera.com/45001academy/free-download/clause-by-clause-explanation-of-iso-45001