2000 is the very lowest. I tend to see it float around 2300~3000.
You can't really split up servers. People want to be with the people they know/friends. Ripping them apart from each other would be hugely negative.
Also, 2 servers would be far too little. Excluding Asura - Merging the other servers together would create a good balance I think. Asura is another matter altogether and should be left as is.