Do you have that kind of flexibility in your controller and drive selection? Sometimes there is a large cost involved in either a larger controller (capability of hosting more drives) or a price premium for larger drives (cost per Gb). Is cost a factor, or is reliability and performance the only drivers involved?
I would never use raid 5 especially something that large. raid 6 would be better with raid 10 probably the best but would cost a whole lot more.
basically the larger the arrays ( total TB's in storage) the higher the chance of a rebuild failure. which is why raid 6 is gonna be your best bet for something that size unless cost is no option then you could go raid 10
raid 6 with 4TB dirves with give you roughly 10 drives for about 40TB total drive storage but when put in raid 6 should give you 32TB ish total storage with a 2 drive failure cushion
If you want your 42TB RAID5 to turn into 36TB of usable storage, then the only possible solution is by using 6TB drives (7 x 6TB).
RAID5 is always built using a number of identical drives (a minimum of 3 drives in the case of RAID5) and uses the volume of 1 drive for parity calculations. So the volume of 1 drive in your example will be 6TB to give you 36TB usable. So the answer is 7 drives @ 6TB each.
If you were to use 2TB drives, you would get 40TB usable from a 42TB RAID5.
If you were to use 3TB drives, you would get 39TB usable from a 42TB RAID5.
I suggest looking at the Hitachi He8TB HDs that are shipping. The most important thing is to look at the length of the warranty for the HDs. The longer the warranty the less likelihood of failure. The larger the drive the longer the RAID rebuild, but the lesser the amount of drives, the lower the likely hood of failure.
we have several NAS's running large arrays of the Seagate Archive level 8tb, not a single issue, I think google or was it cloudflare? proved that warranty does not correlate to drive lifespan, personally when the difference between the cheapest (that we find reliable) and the "recommended" is a doubling in price, I go the cheapest and just have hot spares + just buy more when one does go... of course if the cheapest were to actually be unreliable then that would be another story, but we've been buying the desktop level 4tb for years without issue we're pretty happy.