I just finished my Totodile grind tonight. I'm up to 52 of the Johto pokemon caught. Out of what, 80 max? But the 6 Legendarys aren't in the game yet, Corsola is Southern Hemisphere only, Delibird and Smeargle aren't found yet either, so that's a possible 71.
the hardest to get looks to be the new 3 stage evolution pokemon, Cyndaquils, Chikoritas, Hopips, Larvitars and Mareeps are highest on my hunting list right now since I just hatched a perfect Mareep.
The Gold Medal for Johto is 70, pretty high requirement with only 71 gettable.
Edit: Forgot Heracross, that makes 70 only gettable.