The unevolables, excluded unreleased legendaries, from gen 1 were
The three regionals
Onix *
Hitmonlee *
Hitmonchan *
Lickitung
Tangela
Chansey *
Scyther *
Jynx *
Electabuzz *
Magma *
Pinsir
Lapras
Ditto
Porygon *
Aerodactyl
Snorlax
So, 19 in total, with those starred gaining new evolutionary levels in the gen 2 update.
Gen 2 unevolvables, excluding legendaries, are
Sudowoodo
Aipom
Yanma
Murkrow
Misdreavous
Unown
Wobbuffet
Girafarig
Dunsparce
Gligar
Qwilfish
Shuckle
Sneasel
Delibird (not yet released but counting it anyway as I think it and Smeargle will get their own releases like Ditto)
Mantine
Skarmory
Stantler
Smeargle (not released but counting it)
Miltank
And two regionals.
Which is 21, I think. Out of fewer pokemon (146 v 94, excluding legendaries, 5 in gen 1 and 6 in gen 2)
But perhaps in a future update they will release more Pokémon with different evolutionary stages for those 21, much like Smoochum for Jynx, and Steelix for Onix.