i've seen similar post but skrew it, i lose my mind, and i'll do it again.
anyway, i recently made a 10 player, 19 star map with about 500? planets, i think its more but after a certain point it doesn't matter anymore. it ran good at first but it has slowed down a bit, but considering the vast scope of this map a bit of slowdown is nothing.
PS. there should be auto colonize/auto clear system options and planet development templates to make expansion on this scale more managable. also on the empire tree, what happened to ctrl# groups and being able to collapse info from an entire star? all my planets are always visible which clutters up the tree. not a huge deal but while playing on insanely large galaxies these problems start to come up.