SQL Server infrastructure planning
Is there any general official MS guide for planning SQL Server hardware for instance:


No.  The hardware requirements for 500 concurrent connections is too dependent rate and cost of the queries that the connections execute.  That's why you see this kind of prescriptive configuration guidance from applications (like SharePoint), after they have tested the application at different scales.

If you can break down the workload by type of request, with peak sustained request rates for each, and cost for each in terms of CPU time, Logical IO, and Log IO required you can start to get a first-order approximation of the required hardware.  EG for

SmallRead  - 100 rps,   2ms CPU,    100 LIO,   0 Log IO
BigRead    - 10  rps, 400ms CPU, 100000 LIO,   0 Log IO
SmallWrite - 80  rps,  40ms CPU,     10 LIO,   2 Log IO
BigWrite   - 2   rps, 400ms CPU,  10000 LIO, 200 Log IO


200+4000+3200+800 ~ 8000 ms/s of CPU time, or 8 about cores.

The Logical IO depends on the size of database, size of memory, and cache hit ratio, so it's hard to esitmate.  But having RAM >= 20% of database size typically provides good caching.

And the the IOPS at the log are also hard to estimate, because concurrent transactions piggyback on log IOs.

For Data Warehouse workloads Microsoft does produce prescriptive guidance for hardware configuration in the Data Warehouse Fast Track program that enables hardware vendors to produce reference configurations rated by data warehouse size.   But that's only possible assuming that Data Warehouse workloads are all very similar to each other. 

But if your workload uses a very large database and needs to support analytics, the Fast Track configuration guidance might be relevant.  If you're thinking about using it for a hybrid or OLTP workload, though you'll need to use one of the variants that use all-flash storage.