How many Counter-Guardian does Alaya have? Other than FSN Archer, is there any revealed Counter-Guardian other than him?
According to the Wiki Category "Counter Guardians" on the Type-Moon Wikia there is 4 that are named but there can be more, as Krazer mentions that 7 have appeared to retrain Primate Murder in certain Timelines but we do not know the names of these ones. 2 however were created parody/side works so they might not actually be proper Counter Guardians that Alaya can deploy

Anyway, the 4 Counter Guardians we know the names of are:


EMIYA (Emiya Shirou): we know this one being Archer from Fate/Stay Night. a future version of Emiya Shirou
EMIYA (Emiya Kiritsugu): This one if from Fate/Grand Order as an Assassin Servant who is Kiritsugu, Shirou's adoptive father


  Assassin's True Name is EMIYA, Kiritsugu Emiya. An assassin who was incarnated as an agent of the Counter Force. Originally, he was not a Heroic Spirit. Although he was an individual who murdered many people as an assassin, his existence has not been engraved in the Throne of Heroes. One of the “imitation Heroic Spirits” also referred to as Counter Guardians, and someone who would have never existed in the proper human history. Yet, this "if that might have been" possibility exists now only due to the threat that attempts to destroy human history itself from its very foundation... in the Grand Order case alone. His soul was called out from the circle of deterrence in response to this threat.

Okita Souji: commonly know as Devil Saber who first appeared in the parody manga Koha-Ace. Devil Saber is isn't the same as Sakura Saber and is sorta like an Alter but not listed as one. 


  Devil Saber is a Counter Guardian formed through Sakura Saber's spiritual foundation taking in a Holy Grail filled with Seven Servants. Essentially an Okita Alter, her ascension grants her support from the Counter Force.

TOHSAKA: first appearing as a Master (and not a Servant) in Capsule Servant which was a side game that was released with the PS Vita version of Fate/Hollow Ataraxia. as you can tell by the name she is Tohsaka Rin who became a Counter Guardian in a world where Shirou didn't


  TOHSAKA is a "miraculous" Heroic Spirit born in a world line in which Shirou Emiya did not become the Heroic Spirit EMIYA.
  
  Because of her severe debt, she made a contract with the World to become a Heroic Spirit. Having decided that there were too many Grail Wars occurring across the different dimensions, and because she was a relatively new Heroic Spirit, the World tasked TOHSAKA with destroying the various Holy Grails, reducing her debt slightly for each one eliminated.



However because of the nature of Koha-Ace being self aware and the parody nature of Capsule Servant (ie. how The World is shown to be like Seele from Neon Genesis Evangelion) one could debate whether Alaya can actually deploy either one