There’s no set number of how many keywords you should use in your resume or cover letter. Review the job posting and jot down all of the words or phrases that are repeated throughout. Those are the company’s keywords for this job, so make sure that when you create a customized resume and cover letter to apply with, you’re including those phrases–but proofread your materials and make sure that everything reads naturally, too. You don’t want to stuff keywords and sacrifice quality.