Check out the web pages of the colleges that interest you, especially if you are still in school. Requirements vary from no recommendations considered - to none required - to anything goes - to very specific rules such as limiting the number of letters, or the letters must be from teachers, or recent teachers, or one STEM and one humanities teacher, etc. etc.
If you have been out of school of if the school to which you are applying accepts extra letters (besides those from teachers), your manager who knows you well sounds like the best choice.