Oscar voting begins on January 5th and this year The Academy of Motion Picture Arts and Sciences added 600 voting members this year. Bringing the total to 6,687. The academy invited an unprecedented 683 people to join in an effort to be more diverse. 
Here is a list of each branch and the number of voters they have according to The Wrap:

Actors: 1,158 (+20 from 2015)
Casting Directors: 99 (+13)
Cinematographers: 240 (+13)
Costume Designers: 117 (+2)
Designers: 295 (+17)
Directors: 473 (+79)
Documentary: 277 (+40)
Executives: 468 (+10)
Film Editors: 274 (+20)
Makeup Artists & Hairstylists: 157 (+16)
Music: 279 (+22)
Producers: 504 (+21)
Public Relations: 389 (+1)
Short Films and Feature Animation: 479 (+78)
Sound: 456 (+19)
Visual Effects: 383 (+24)
Writers: 406 (+14)
Members at Large: 233 (+17)
Total voting members: 6687 (+606)

It’s important to note how many actors vote.  Obviously, actors are going to vote for movies that showcase acting. They also like films with a lot of actors in them. They aren’t going to vote for films that use CGI that could replace them. They vote for who they like and who their friends are. 

Most of the actors are working actors and don’t have time to see many of the movies in contention. They will watch what they have heard of or whatever the family wants to sit down and watch over the holidays. They are not required to watch every film before voting or any film at all for that matter. It’s far from a perfect system.