The owner of a computer can use it as he sees fit.
If a software product is to be used only on Windows Server, it will just not
let itself be installed on a client version of Windows.
This is true for almost all Microsoft products.
In short, any software you have legally installed can be used as you see fit,
according to the principle of "fair use".
You may support as many SQL Server users on the Windows 10 computer as
the hardware and software will successfully support.
Microsoft already took care of the SQL Server license by limiting the number
of allowed concurrent connection for a Windows client version, so even
if you tried you cannot exceed the specified maximum without illegal hacking
of your Windows setup.
EDIT: Many persons wrongly thought that I was advocating software piracy.
Please note that I said "legally installed", meaning bought for that computer
and used on the licensed computer without hacking.
EDIT2: Although many people have upvoted my answer, there are more
downvoters. I'm not a lawyer, but I have been following "fair use" decrees by
US courts, so I would like to include here some of my answers to criticism.
The EULA of a product may contain new restrictions on the use of the product
Wrong. The EULA cannot add new conditions of sale that were not communicated
previous to the sale. Once the EULA is displayed after the sale, the product is
no longer the property of the manufacturer, so he has no authority to rewrite
the contract.
Clauses such as "Continuing means agreement with" are invalid in the EULA.
In contradiction, clauses such as "Breaking this seal will void guarantee"
do work, because these are conditions on future services.
I installed a 4-core product on a 24-core computer and it's using all of
them. Am I in violation?
No. The vendor of the product cannot force you to buy a new computer!
The most he could have done was to limit his product to using 4 cores,
and he would have been within his rights. The user cannot be held responsible
for such a defect in the product.
I installed a 10-connections product in my office network of 20 computers
This is exactly the same case as before. The installed product can limit
its own functioning, but not to ask you to fire 10 employees.
Microsoft was in a position to enforce its 20-connections limit on
client Windows,
by the brute-force method of limiting the number of connections it allows
to a Windows client computer. Other developers must enforce their own
limits inside their own product, but certainly cannot force the client
to make any changes in his existing hardware that they didn't themselves
manufacture.
You should be able to get a Windows Server virtual machine to use as a web server at a reasonable price from a hosting provider. Look around a bit more. – Andrew Morton – 2019-10-28T14:32:39.653
1Microsoft makes the Windows license available publicly - did you by chance consult that, as the information you're requesting, which @amtoo quoted a portion below, is contained within it. While there is legalese within a license/warranty, Microsoft has tried to make their licenses more consumer-friendly to read. – JW0914 – 2019-10-28T17:46:34.747
If you are only using for yourself, and stay within the device limit, you are fine. Firewall out the rest of the world. Anything accessing shared folders also counts. This is actually way more generous than XP, which I think had 11 as a limit? – mckenzm – 2019-10-28T23:51:51.990
1Use Apache and Postgres and this licensing nonsense goes away. – Flexo – 2019-10-29T10:27:00.090