Introduced in X and Y: 26
Introduced in Omega Ruby and Alpha Sapphire: 20
All Pokemon total capable of Mega Evolution: 46
Charizard and Mewtwo each have two different Mega forms. Therefore, there are 46 Pokemon that can Mega Evolve, but 48 Mega Evolutions. I didn't include Primal Reversion, as they are not technically Mega Evolutions.
Hope I helped!