A customer wants to rent a dedicated Server for their site. They do not need more than SQL Express 2017, for the time being, but they have been offered a dual Xeon system.
Does SQL Express install and work on a dual CPU system, albeit using just one CPU, or do they have to ask for a single CPU server?
Sorry but I haven't found an answer to this specific question.
Will it install?
Yes.
Will it work?
Yes.
Will it use all of the horsepower the server has available?
No. The limitation of the Express Edition -- "single physical CPU or four cores" -- means that SQL will detect more than one CPU and will set the others to offline. You'll see that in the results of select * from sys.dm_os_schedulers both as "VISIBLE OFFLINE" in the status column and as "0" in the is_online column.
So you and your customer can do this, but you'll be wasting some money. The recommended approach all depends on what the price difference is between the dual-CPU system and a similar single-CPU system. If it is a small cost difference, and there is a strong possibility that they will upgrade to Standard Edition or Enterprise Edition soon (after all, you do mention that Express Edition is enough "for the time being"), you might want to advise them to start with the dual-CPU system to enable making that future edition upgrade without also having to deal with a server upgrade. If the price difference is not insignificant, go with the single-CPU system so you don't waste the cost of the second CPU.
SQL Server Express Edition will work fine, but it's overkill to dedicate a whole multi-socket server to it. It will only use 4 cores and about 1GB of memory.
But that's OK, since this is 2020 and you shouldn't be running application workloads directly on bare metal servers in the first place. Install a hypervisor, like the Hyper-V server, and then provision a VM to run SQL Server Express Edition. The VM will be portable to other servers in case of a failure, and you can provision additional VMs to utilize the server's resources not needed for SQL Server Express.
No .. that wont work as your client/customer expects.
https://www.mssqltips.com/sqlservertip/5527/deciding-to-use-sql-server-2017-express-edition/
Single physical CPU or four cores are allowed
As you mentioned the client aims at Dual Xeon systems .. there are no Xeons with only 2 cores to my knowledge (except maybe OLD ones fresh after the stone-age).. So either he uses a single Xeon of 4 physical cores - or has to use a "grown up"-Version of MSSQL server
