Your prospective employer is fishing for an idea of how many jobs you've applied for. Though you may want to seem eager about the role or being "all in" for the company, refrain from implying that you've only applied to this one job. Your interviewer may not feel you're serious about securing gainful employment, or may not believe you. Don't overcorrect and gush about how many companies are vying for you, either. Let the interviewer know that you're looking seriously into several options, but haven't yet decided which is best.
