SQL already handles all that at the server level. You are not going to be able to make better decisions about processor resources then the DB. It'll use as few or many cores as it sees fit based upon the runtime configuration settings that the server has. Unless your DBA has changed it then SQL should use all the cores on the machine (or licensed) automatically. Refer to this article for more information. You can read the official docs here.
You can limit the parallel options for a query using MAXDOP. However this only can be used to reduce it, not increase it. So if the server is configured to use 4 cores you can limit your query to 1-4 but cannot make it use the remaining 4 cores.