Pair as in 2?
Oh, I also threw in a pair of nvme drives for caching. It noticeably speeds up read speeds. You can use non-synology drives but it'll limit you to using for caching as opposed to storage but that should be fine seeing as you will have 4 bays for storage anyways.
These are reliable and very cheap at the moment. You'll need a pair of them.
https://www.amazon.com/gp/product/B08X4Z4R1Y/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
Discussion
Yep make sure to get two of them. I think it works with just one but at this price I just went for two and mirrored them in the off chance of a power failure or whatever during read/write.