Board Thread:Questions and Answers/@comment-175.139.127.141-20130204094534/@comment-917693-20130207222701

not sure what you mean exactly. i guess it's difficult to determine a certain number, but many original songs are placed under the NNI (ニコニコインディーズ) category. on the wiki, they should be marked as originals though.

(but rainbow girl is not a original i think.)

and being an original doesn't consequently mean, that it has to have a (moving) PV, e.g. yuikonnu's "Phantom".