If there was multicore support, what limit is there to hidden service or relay bandwidth? Given that a single cloudflare server supports at least six gigabits of bandwidth (although it's likely reached sixty gigabits with their latest generation), would there effectively be no limit (given that total advertised bandwidth is 120 gigabits / second)?
I'm just asking if Tor can operate over multiple ethernet ports.
No. The solution is to run multiple copies of Tor, in which the configurations of each specify different addresses and port numbers.
Most likely your processor will slow you down because a TOR relay needs to do much cryptographic operations. Intel CPUs with AES-NI support will push that limit higher but still at some point it can't go any faster. Using this command shows how much data your CPU can process:
openssl speed -evp aes-256-cbc