As-salamu alaykum
Ahad means One. This is basic Arabic. It is also the first number! I am a little surprised that this would even be questioned on the basis of the text of the Quran.
Of course, if I am going to say choose from one of them, then the main group will refer to many. However, the subject will be one. For example, we make no division between any one of His messengers (2:285).So one of them wishes he might be spared (2:96); if one of the two women errs (2:282); It was accepted of one of them (5:27); let not one of you turn around (11:81); two men, one of them dumb (16:76) etc. Please also note the preposition (minkum) which implies the main group from where one is referenced.
So if I say God is one, I am not referring to any group. I am stating He is One!
I hope that helps, God willing
Joseph