As of X and Y, there are 719 Pokemon confirmed by official sources. However, two more Pokemon exist in the code of X and Y, which would bring the total to 721. These Pokemon (Hoopa and Volcanion) are not officially confirmed though, and cannot be obtained in any way besides hacking. There are also 28 confirmed Mega Evolutions in X and Y (and two more that exist in the code, Mega Latias and Latios), which would bring the total to 749, 751 or 753 depending on what you count.
Omega Ruby and Alpha Sapphire do not add any new Pokemon, so the total remains at 719 or 721 including the ones found in the code. There are currently 20 Mega Evolutions confirmed for the games (21 including Mega Hoopa which was found in the games' demo's code), which would bring the total amount of Pokemon and Mega Evolutions to 770 (including Pokemon in the code). Mega Latias and Latios are also confirmed for the games, and are part of the 20 mentioned earlier.
So basically:
- If you count only confirmed Pokemon: 719 Pokemon and 48 confirmed Mega Evolutions, total 767
- If you also count Pokemon found in the code: 721 Pokemon and 49 Mega Evolutions, total 770
