Limited Number: There are only 7 Servants allowed within the competition, once all 7 classes have been filled, no more are allowed, unless something happens due to decision for a plot twist.
Command Spells: When a Servant's master gives them an order that the Servant doesn't wish to do, the Servant must comply with the order regardless. This only happens three times, afterwards the Servant has no need to listen to their master's orders.
However, any order must be obeyed. For example, if a Master ordered his Servant to have sex with him, she would have no choice. Another example, if a Master told their Servant to kill themselves, the Servant has no choice. Though it would be unwise for a Master to do that, as the Master would lower his chance of victory considerably.
Noble Phantasms: Having a Noble Phantasm isn't required, however it might hamper your abilities unless you character is known for a legendary technique.
Actual person: A custom made servant must be someone in actual legend, myth or history. For example, you can make a character such as Perseus or Morgan Le Fay. Another thing is, the character cannot be a god, like Zeus or Quetzalcoatl. Demi-gods are permitted though.
Hidden Identity: A Servant's true identity is hidden, to make sure that the weakness of the Servant isn't exploited. All Servants have something which will reveal who they are, usually in the form of their Noble Phantasm. Knowing who the Servant really is can possibly tell someone how to defeat the Servant. It is impossible to tell a Servant's identity by looks alone, unless two Servants knew each other in their own lives. For example, if King Arthur and Mordred were two different classes, they would identify each other rather easily.
