Mining?

I think it's better don't use PC (CPU Mining), It's useless. In a best condition, you can mine XMR(Monero) in a very low rate.

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If you like to mine, you can use a browser based mining script with https://coinhive.com .

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It's depend of mining software's configuration that how many core use to mine. Generally, if a mining soft run secretly on our system, use half of your processor and if you want to use a software, you can select the number of cpu core.