How many SELECT/UPDATE/INSERT Operations can you run on a 4 core / 8 GB RAM server? Assume you are working on a web project with html/css/js frontend with php in backend. How many SQL Operations (executed by mysqli for example) can do do with 4 core / 8 GB RAM server ? The database is running on a dedicated server.

I am only talking about some simple SELECT/UPDATE/INSERT Operations with maybe 2-3 subselects.

I guess the higher complexity level of the query the higher the (cpu/ram/io ?) ?

Is there some kind of "table" like 2c/4GB -> 10000 OPS/second , 4c/8GB -> 50000 OPS/second ?

I alreay took some tries in google but i only got links to specific database hardware requirements from ibm or sap hana. 

Is this just a stupid question or can't you make a "general" recommendation. Of course i know that you cant make an exact recommendation but maybe a roughly one ?

Thanks in advance.
The maths behind it is far more complex than you seem to be implying.

Sub-selects count as queries in their own right, so a query with 3 sub-selects counts as 4 queries.

Size of the data matters as does indexing and indexability of every query.

If a query is returning 100,000 rows, it'll be orders of magnitude slower than a query returning 1 row.

If a query needs to check 1M rows to compute the result, it'll be slow regardless of the fact that it returns 1 row.

The "unit" of complexity is not a query. The CPU in question might get you 10,000 queries per second of one query on one data set, but 0.001 queries per second of another query on the same data set or 0.00001 qps of the same query on a different data set.

There is almost no limit on how computationally complex "a query" can be.