Hi Veep, I'm autistic and decided to see how many Pokemon cannot be obtained in a given generation. Main games only. I.e. I counted up what couldn't be obtained in R/S/E/FR/LG and excluded the GCN games. So:

Gen I - 1 (Mew)
Gen II - 19 (Kanto starters, fossils, Kanto legends, mythicals)
Gen III - 6 (Lugia, Ho-Oh, mythicals)
Gen IV - 13 (Regi trio, Regigigas, mythicals)
Gen V - 100 (A lotta shit)
Gen VI - 15 (Mythicals)

Pic not related