yes, multi-thread works on windows.

but you could not use workers in windows because workers implemented based fork and it's not supported by the Windows system.

Due to python GIL multi-threaded server could not utilize all the core of CPU.
but the worker(Preforked Server) help odoo to utilize all the CPU core of the machine. that's why it's recommended to use workers in production.
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what is the best proportion of workers/core and cron-workers/core?​
​it highly depends on your requirements what kind of cron job are you running. by default, odoo create two cron thread/process to handle cron job in most of the case it's sufficient.

but check your log and analyze if both cron thead/process keep busy all the time then you can add more.
(early symptoms of all busy cron are your scheduler didn't run on time as you specified in code like you specified run every 5 minuter and it takes more time than that)