I think it's more like...20% went to the other side (mainly big guilds like Seigi, Destined, Renai, MotE), 30% were like waitwtfzomgimoutofhereoridontknowwhatsgoingon and 50% stayed.
Just Anima, before Debs died.
Lolololol.
I know right 
Yeah. Not alot of people actually went to tRO. Silver was scheming and asking around if people would follow, but he couldn't like.. spread it around that much, since he was still hired by Kanemi, so I guess that's why not alot of people went there, or they just.. didn't care.
*nods* I recall the first days there were ~300 online, but some people were interserver trading back then as well. It was mostly the bigger guilds who were more informed about things (due to private forums or something I guess) who switched. Then a lot of people who didn't wanna bother switching and a lot who were just clueless. And then a lot who just quit both servers after all the bullshit, haha.
But yes, I recall on the IRC channel they marked the peak at around 3k or just a little below it, not too long before Deb/Pmak/whatever died. aRO was left with...1k or so after the split? Then roughly half of the original players probably quit.