There are typically 7 mages that participate and so 7 servants as well. Not anybody can participate in this war as well, since the mages participating must have great power to provide their servant with mana. While this seems simple and does not require brutality on the parts of the mages, the Holy Grail War is anything but and the mages tend to not follow the rules set by the mages council.
These servants are typically are great heroic warriors who have sacrificed themselves when they were alive and now serve as a spiritual servant, only called upon when a Holy Grail War ensues. What occurs in each season is a war over the wish granting Holy Grail, which can only be won by 1 servant and their mage, thus setting a playing field for the mages to fight to the death with their servants, not each other. The Fate Series while they all take place for the same reason, are set in different times with different mages and different servants.
The series I will analyze in this blog post will specifically Fate Stay Night and Apocrypha as they have many more LGBT+ characters, heavy emphasis on the T. While they were released in a different order than the story unfolds, the typically order in which people watch them are Fate Zero, Fate Stay Night and finally Fate Apocrypha.