How many people does it take to make a modern video game?

A team of over a hundred people can be employed full-time for a single project. however i think it depends on the type of video game.

This can be a very broad number, and the answer can be as little as one person. For example, this gamesradar link lists ten video games that were developed by just one person.

https://careerkarma.com/blog/games-made-by-one-person/

Some games are so large that they require large studios (sometimes multiple studios) with many people working on the game. In the case of larger open-world games such as "Grand Theft Auto", it took over 1,000 people to create the game.

http://www.gamespot.com/articles/rockstar-more-than-1000-people-made-gtav/1100-6415330/