Fibre optics single core or two core for internet

Hi,
Apologies for my username, i am not that smart My question is What are the best practices followed by ISPs when they bring in bandwidth from their upstream provider via fibreoptics cable ? Eg, if i want to purchase 10g bandwidth from upstream provider how many cores of fibre i need to arrange ? And what kind of device with that capacity i need to purchase ? Kindly englighten me on my question. Thank You...