Playing online games uses on average of 400 MB to 600 MB of data per hour which is less than 1 GB of data.
If you have a limited internet data plan then you will likely need to watch your data usage when playing online games.
However playing a few online games is unlikely to use a whole lot of data and you may or may not go over your data plan.
Playing online games also uses less data per hour than watching and streaming Netflix movies and shows.
If you have a limited data plan such as 1 TB then you could play several hundred online games before you reached or exceeded your limited data plan.
If you have an unlimited data plan then it's not an issue.