Is it possible for one Master to have multiple Servants? I'm reading the Fate/Stay Night visual novel. Early on, in the Kotomine Church, you can reach a bad ending by choosing to have Shirou give up his Command Spells and withdraw from the Holy Grail War as a Master. He goes outside to sever his contract with Saber, who says that normally she would kill a Master who abandoned her. Shirou asks if she'll form a contract with another Master, and Saber says that Archer's Master (Rin Tousaka) should be able to bring out her full power. Upon parting with Shirou, she goes into the church, presumably to form a contract with Rin. 

Also, in the 2006 Studio Deen Fate/Stay Night anime (I haven't reached this far yet in the visual novel, but I assume it's similar), 


   Kirei Kotomine commands both Zero Archer (Gilgamesh) and Stay Night Lancer (Cu Chulainn). I assume he was officially the Master of at least Lancer, since unlike Gilgamesh, Lancer would have required a contract to stay in the world. 


But I can't remember if it was ever stated that he is the official Master of both, or if 


   Gilgamesh was maintaining his existence with his own mana, and just stuck with Kirei for his own reasons.


We do know that it's possible for a Servant to have two Masters, as, for a time, both 


   Kayneth and Sola-Ui are Masters to Lancer 


in Fate/Zero. But are the scenarios above instances of Masters with two Servants? Would Rin have been able to make a contract with Saber while also maintaining a contract with Archer, or was Saber relying on her knowledge that Rin wanted her as a Servant, and would cut off Archer if Saber became available? Was 


   Kirei the Master of both Archer and Lancer, or only of Lancer, with Archer a free agent?


If the answer is yes, do we know how the situation with Command Spells works? I assume this isn't a problem for 


   Kirei, as he gained all the leftover Command Spells from previous wars when he found his father dying from Kayneth's bullet in Fate/Zero.


But would Rin have gotten an extra three Command Spells if she made a contract with Saber, or would she have to save the one she had left as a sign of her control over both Saber and Archer?

I'm still early into the visual novel, and I haven't yet seen the last four episodes of Fate/Zero, so please use spoiler tags as needed.
It should be possible. However most of the examples i can find are non-human.

In Fate/Apocrypha we learn that the entire Red faction, save Mordrid, is under Shiro Kotomine's control since their Masters aren't in Trifas and he has been working on taking control; I remember reading that he has their Command Seals. The reason he can do this is probably because


   He is the Ruler from the 3rd War as such a Servant with a connection to the Grail


As zibadawa timmy mentioned, in Unlimited Blade Works Caster had Assassin, Archer and Saber as her own Servants. However with Assassin she was a Fake Master, but with the latter two, she became a proper Master after using her Nobel Phantasm Rule Breaker to null their contracts with their Masters. 

The only Human example is with Sakura in the Heaven's Feel Route. She has a contract with Rider (from the beginning) but she also, when she accepts Angra Mainyu, is the Master of Alter-Saber and Blind Beserker. However there is the possibility that Sakura was in fact a Servant herself with Angra Mainyu as the Master

The above three are evidence of how a "Master" can have multiple Servants under the Fuyuki system; there are examples outside that.

In the Moon Cell, the Alter Egos created by BB are her Servants. However BB is an AI and the Moon Cell/SE.RA.PH operates differently than the Fuyuki Holy Grail which is supposedly what it used to develop the system to determine who was worthy to reach the Moon Cell's core.

There is also Manaka Sajyou from Fate/Prototype. She not only is the Master of Beast but she also ends up summoning the other 6 Servants. However


  When she died eight years ago, she became something of a half-zombie due to the power of the Holy Grail


As such, one can't really consider her a normal human.



When it comes to Command Spells, if a Master contracts with another Servant they are granted a set. However this is only seen with Caster. 


  Upon using Rule Breaker to steal Saber's contract, she appears to be branded with actual Command Spells. 


Source: Special Command Spells - Imitation Command Spells

We can assume that it was the same happened with Archer and we can assume the reason is the same as if a Master-less Servant found a new Master (in the case of Caster).