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http://rotcex.com/
The only "Exchange" i know of right now, for RVN.
because it launched just one month ago.

It's 21 Billion.

Not many exchanges for this relatively new coin, yet. You can mine it with your GPU though, here's how:
https://steemit.com/ravencoin/@atheron/mining-ravencoin-using-ccminer-with-nvidia-gtx-1070-on-suprnova-pool