Seiko has released a lot. I don't have the exact count, but my estimate would be 440~470 songs.
If I add up everything that I have then the library easily exceeds 500.
Problem is that there are duplicates and karaoke tracks. I didn't include remixes though.
http://www2a.biglobe.ne.jp/~seikofan...songs_400.html
The above web hasn't been updated since early 1997. The song count was already 350 back then.