You'll have to find some kind of balance, ad it js a game of chance. You are always limited by number of slots in your server and current largest drive size. Then you are trying to balance price, speed, and durability.

For exampl, let's say maximum amount of drives is 10, and maximum manufactured size is 50 TB. You probably don't need 500 TB of storage that is in no way durable (if a drive dies, all the data on it is lost) and on a single server.

Death of drives is almost certain, two drives dying at the same time is quite low, so something like RAID 4, 5, 6, or 10 is a great start. Depends on how much storage you want, and then partition it accordingly. If you want 20 TB, you can do 4x 8TB in RAID 5, which yields 3x 8TB (=24) of effective storage.

Adding new drives is easy, and you are are always wasting just one drive. Then it depends if you want to sacrifice more space for more durability and switch to RAID 6 later on.

If you want even more storage, you can buy a micro server like ODROID H4+ and use it as network connected storage.