I'm going to count the frequency of every type, so uhh, yeah.
Grass: 3
Poison: 3
Fire: 5
Flying: 6
Dragon: 9
Water: 5
Psychic: 10
Ghost: 3
Normal: 4
Bug: 4
Dark: 6
Rock: 3
Fighting: 7
Electric: 2
Steel: 6
Fairy: 5
Ground: 4
Ice: 2
What I found is you can make a flying type, Dragon type, Psychic type, Fighting type and steel type.
Flying megas: Mega zard Y, Mega Pinsir, Mega Aerodactyl, Mega genwun birb (Pidgeot), Mega salamence and broken birb (mega ray)
Dragon megas: Zard X, Ampharos, Latios, Latias, Garchomp, Sceptile, Altaria, Salamence and Rayquaza
Psychic megas: Alakazam, Mewtwos, Gardevoir, Yoga instructor (medicham), Lati twins, Slowbro, Metagross and gallade
Fighting megas: Mewtwo X, Heracross, Blaziken, Medicham, Lucario, Lopunny, Gallade
Steel megas: Scizor, Mawile, Aggron, Lucario, Steelix and Metagross
Because I'm dumb, I definitely missed something, so uhh, yeah.
Source