Not only that. Number of jobs > number of open job postings > number of actual open jobs > number of job openings available to external applicants > number of job openings that are actually trying to get filled (as opposed to just fishing) > number of job openings actually likely to be filled.

But your right, number of companies is very relevant, since you interview for a company not a single job opening. 30,000 jobs at 20 companies represents 20 chances, not 30,000 chances.