Remember, TueNAS uses ZFS as the filing system and it's a beefy redundant system. You can't have redundancy if you use only a single drive. You can Google search for definitions of how RAIDZ works and get a better understanding. For 10 TB of storage and to just hold video content and no real important data (important data to me is financial data, family photos, etc) then a simple mirror of two 10TB drives would work but I would not recommend it. It takes a long time to resilver (rebuild the new drive) if it's that large and if a failure of the only good drive with all the data occurs, then all the data is lost. Hence why we have backups. So this is the minimum I would recommend, but this would give you about ~8TB of usable storage so you may need to go up to a larger hard drive size.
My personal (key word here is personal) preference would be four 8TB drives in a RAIDZ2 configuration = 14.6TB (usable) - 10% (1.5TB) = 13 TB of real storage space. If you drop down to 6TB drives than you are back to ~8TB of storage space with four drives. I do prefer RAIDZ2 because I can lose a drive and replace it, if a second drive fails during that replacement then all my data is still in tact. It's truly a pain in the rear to rebuild a pool and restore all your data. It's time consuming. You could reduce the pool by 1 drives in favor of a RAIDZ1 for a one drive failure tolerance if you desire.
Now lets get something clear... If you start with a three drive RAIDZ1, you cannot add another drive to make it RAIDZ2. Just as you cannot add a single drive to increase the capacity in a RAIDZ without losing the fault tolerance. If you do not understand about these things, you need to read up on them, it's important. I can't tell you how many people had a ton of data and a well configured pool and then added a single STRIPE drive to the entire thing and put it all at high risk.
Also, just to push this one last time, if you don't already have good hardware, make sure you buy good hardware. Do not half-ass it or you will likely be buying more hardware soon down the road. There is a Resource discussing hardware and if you have not already done so, I'd read it. The good thing is you can take your hard drives from one computer and place them in another computer and run it without issue. Got to love this stuff.
Last thing, make sure you do not purchase SMR drives. If you do not know what that is, Google it.
Good luck.