No matter the galaxy size, max tags per precursor is 100.
So, in a 1000 star galaxy you will have empty space even with 9 precursors, while in a 200 star galaxy you will have no holes even with only 2 precursors.
That meshes with my experience - playing singleplayer on huge I often dont get all my precursor sites until 60-100 years in, while in medium maps (when playing with my friends), I can get all the sites much faster.