Depends on the resolution (SD or HD) and what compression they use (H.264 or another MPEG-4 variant or even a less compressive codec).
Anyway, since an MPEG-4 SD movie normally takes less than 1 GB away and an MPEG-4 HD movie normally takes 5 GB away, you can get up to 6.000 or 8.000 SD movies onto a 6 or 8 TB HDD and 1.200 or 1.600 HD movies onto a 6 or 8 TB HDD.