Hi i was planing on buy this lan li PC-D8000 and this can 20 hard drives so my plan is to buy 8tb red drives x20 and use this as my back up drive storage. I am wondering is their any limitation that I might face as I am new to nas world.
There's no practical limit to the number of hard drives that FreeNAS supports. As you've already been told in your intro thread, you really need to check out the manual and some of the sticky threads to get an idea of FreeNAS's capabilities.