RAID, not sure what Type ??

See the following article for an exhaustive discussion:

http://www.techrepublic.com/blog/the-enterprise-cloud/raid-6-or-raid-1-plus-0-which-should-you-choose/

Summary:

When it comes to a choice between RAID 5 and RAID 6, from a data protection standpoint, RAID 6 is the better choice.
Although cost is always an important consideration... in most situations, if given the option, I'd choose RAID 10 (data striping over mirrored data sets) over either RAID 5 or RAID 6.

For complete explanation(s), please refer to the indicated article...

I'd say it depends on the speed of the network, utilization of systems, and the speed of the hard drive used.  Are you using SATA 7.2K?  SAS 10K?  SAS 15K?  SSD?  NVME?

Certainly never use RAID 5. That is an obsolete RAID and risky compared to others.

I'd suggest Several RAID 1 arrays, You can then have one VM on array 1, and the other VM on array 2 (or even have 2 other separate arrays so you can split the OS arrays from the data arrays). As they are separate from each other they don't bite each other.

Will you be using a hardware RAID  controller?
If so ,get one with on board cache,do not use any of that fake onboard crap or you will be sorry.(especially with Vmware).

As far as I know ESXi only supports real RAID controllers. The fake ones I doubt are on the HCL.

As rindi said, stay well away from RAID-5. Use RAID-1, RAID10 or RAID-6

5TB is by todays standard, a small storage system, what you need to be looking at, is what kind of useage profile you will need ie MB/s or IOPS, and if a set of SSD's (or Flash or whatever) will be a better bet, over spinning disk, especially in the long term (cost/performance analysis)

Asking what RAID level w/o also asking about what HDDs/SSDs to use AND what controller, and BUDGET and I/O load is like asking what fuel to buy for an engine, and not even saying if it is for an airplane, submarine, car or train.

You're going to get the wrong answer w/o some constraints.

The RAID board is this one: Smart Array P440ar with 2GB FBWC
The server is an HP ProLiant DL380 Gen9
The files on the server will be mainly AutoCAD, 3D Drawings, and Adobe Photoshop files.
About  20 staff will use if for this.

Does this help?

Based on that, your I/O is going to be read intensive, large block, and a rather small workload.  Why?  Because the nature of those files is that people load them, do crunching for quite some time, then save them.

Go with 4 x 3TB HP SAS drives in a RAID10.  You're throwing money away if you consider SSDs.  Make sure you get HP branded drives because you want the HP firmware.

It'll almost certainly have 8 * SFF disk bays so 3TB disks won't be available. There are 2TB 7.2k SAS SFF available though, you would need 5 of them in RAID 6 for your 5TB requirement. Don't put VMware on them, it's much more convenient to put that on a microSD card.

Wouldn't a 6 disk RAID 10 be better than a 5 disk RAID 6 for this ( Faster and more redundancy) but drawback is speed or maybe that would not very noticeable.

Faster for write but HP Smart Array controllers don't do load balanced reads very well on large sequential files so you would only get 3 * read speed for 6 disks in RAID 10 whereas you would get 5 * read speed for 5 disks in RAID 6.

Debatable whether 6 disks in RAID 10 has more or less redundancy than 5 disks in RAID 6. There's a double-mirror RAID 10 option but that would need 9 disks and you don't have enough bays for that.

You'll want to use RAID 6 or RAID 10 for top performance, based on your workloads.

Do not install VMware ESXi in the Spinning Rust, but use a thumb drive (flash) or SD card.

Also download the OEM ESXi version from HPE, and not VMware.

I think you will find RAID 6 will be suitable for your needs.