[@baraquiel]

Noble Phantasm:
are "crystallized mysteries", powerful armaments made using the imagination of humans as their core, and weapons or abilities owned by Heroic Spirits. They are the embodiment of the ultimate mysteries of a hero that symbolizes one's existence through historical facts and anecdotes. They can be physical weapons, such as swords, lances, and bows, or support items, such as rings and crowns.[1] They can also be abstract concepts, like unique abilities, unique means of attacks, curses, and changes to the environment and its physical properties. When summoned as Servants in the Holy Grail War, they are the trump cards of the heroes that allow them to overcome others in battle.

Here is the info on Command seals:
http://typemoon.wikia.com/wiki/Command_Spells

[@Blueflame]
for right now let's say one servant and one master for everyone. This may change depending on how many people we get interested in the RPG