The common floor people cite is 150 to 200 hours, though most successful applicants we know personally had several hundred, and plenty of gap-year applicants had well over 1,000. There's no hard cutoff, but showing up with the bare minimum rarely tells a compelling story on its own.
Quality and longevity matter more than the raw hour count. Admissions committees want to see that you've spent enough sustained time around sick people to genuinely understand what you're signing up for, not that you cleared a number. Two years at one clinic tells a far better story in an interview than ten disconnected one-off shifts, even if the total hours end up similar.