When I bought my ReadyNas214 on their website it said it can hold a maximum of 48TB.
The NAS itself has no known capacity limit. The website and datasheets use the maximum compatible disks that are available at the time they are published. They also assume that RAID redundancy isn't being used (which pumps up the specs).
At the time of publication, the largest disks in the HCL were 12 TB - and 4x12=48 TB. Today the largest disks on the HCL are 16 TB, so if they updated the website and datasheets they'd say it supports 64 TB.
Although there are situations where people don't use RAID redundancy, personally I think it is a good idea. A major benefit is that you can upgrade storage w/o needing to restore your files from backup. A general rule of thumb is that the actual volume capacity is computed with "sum the disks and subtract the largest". With your configuration, that works out to 18 TB (16.37 TiB).
What is the best way to maximise this in future? Also keeping my costs down as well.
Overall, I think you are in a pretty good position with downstream expansion costs. As your storage needs grow, you can just upgrade the each remaining 3 TB disks to 12 TB - which would double your storage capacity over time. Upgrading one disk would increase your storage by 9 TB, upgrading both would increase your storage by 18 TB. As I explain below, upgrading your 3 TB drives to 16 TB (or larger) when the time comes might also be a good path. But I suggest waiting until you actually need more storage, and analyze the costs then.
Here's more on how I look at it myself. Sorting out the most cost effective expansion path generally requires some analysis of disk prices and the growth rate of my storage. Basically, I plot out a couple of upgrade paths, and cost them out before I buy.
One factor is how to measure cost. You could look at only the total dollar amount, but personally I find it more useful to also look at dollars per TB gained. And also I try to think one step ahead - what would I have to do the next time I need to increase the storage.
This approach generally leads to upgrading the smallest drives first, and also increasing storage in larger steps. Replacing working 12 TB drives with 14 TB ones makes little economic sense.
If I am setting up a 4 to 8 bay NAS from scratch, then I find that it makes sense to get larger disks, and leave at least one slot empty. Filling empty slots is cheaper than upgrading disks to larger sizes, since I'm not removing storage that works. If I don't have any empty slots, then it is most economical to upgrade the smallest disks first. I usually upgrade some drives with larger sizes (instead of upgrading all of them to a somewhat smaller size).
Also when a drive fails (or is replaced because of it's age), then I look at upgrading the size, and not simply replace it with a like disk.
To make this more practical: let's assume that you started off with 4x3TB (a 9 TB volume) in your RN214. Let's also assume you are using NAS-purposed drives (not enterprise class). Costs below are USD, and based on current Seagate Ironwolf pricing. WD Red Plus drives should give similar results (though of course you'd want to use disk prices in your country).
You could have gotten your current 18 TB volume by upgrading all 4 disks to 6 TB. With today's pricing, that would have cost $624 - or about $70 per TB gained. Upgrading two drives to 12 TB was a much better choice - giving you the same volume size for $558 - or $62 per TB gained. And the 12 TB path makes future upgrades will be less expensive. Upgrading a third 3 TB drive to 12 TB would increase your volume size by 9 TB, and would cost about $280 US - so the cost per TB gained is about $31.
As a hypothetical, upgrading both your 3 TB drives to 16 TB would cost about $685 USD, and would give you a volume size of 40 TB. That's a gain of 22 TB, so the cost per TB gained also works out to about $31. If you needed a big increase in storage right away, that would be a reasonable strategy.
A "split the difference" strategy is to upgrade one 3 TB disk to 16 TB the next time you need more storage. Your volume would only grow by 9 TB, so the cost per TB gained for the first upgrade goes up to $38. (Another way to look at it is that you'd spend $62 more up front over a 12 TB drive. But the next 3 TB to 16 TB upgrade would gain you 13 TB, so you'd recover the extra up-front money later on.) Similarly, if a 12 TB drive fails you might choose to replace it with a 16 TB drive. I've sometimes used this strategy - particularly if I have older drives in the array that I think will need replacing in the next year or so. Another scenario is if I am replacing a failed drive, but see a need for some expansion in a year or so. Since disk prices fall over time, I generally won't spend a lot of money now to avoid costs I think are several years out.
