John / Jane foi o único viajante do tempo sendo manipulado pela Agência?

No conto, embora você nunca "veja" mais ninguém viajando no tempo, é muito strongmente implícito que o narrador é apenas um dos muitos agentes de tempo:

2300-VII-12 Aug 1985-Sub Rockies Base: I woke the duty sergeant, showed my I.D., told the sergeant to bed him down with a happy pill and recruit him in the morning. The sergeant looked sour but rank is rank, regardless of era; he did what I said—thinking no doubt, that the next time we met he might be the colonel and I the sergeant. Which can happen in our corps. "What name?" he asked.
I wrote it out. He raised his eyebrows. "Like so, eh? Hmm—"
"You just do your job, Sergeant." I turned to my companion. "Son, your troubles are over. You’re about to start the best job a man ever held—and you’ll do well. I know."
"But—"
" ‘But’ nothing. Get a night’s sleep, then look over the proposition. You’ll like it."
"That you will!" agreed the sergeant. "Look at me—born in 1917—still around, still young, still enjoying life." I went back to the jump room, set everything on preselected zero.
2301-V-7 Nov 1970-NYC-"Pop’s Place": I came out of the storeroom carrying a fifth of Drambuie to account for the minute I had been gone. My assistant was arguing with the customer who had been playing "I’m My Own Granpaw!" I said, "Oh, let him play it, then unplug it." I was very tired.
It’s rough, but somebody must do it and it’s very hard to recruit anyone in the later years, since the Mistake of 1972. Can you think of a better source than to pick people all fouled up where they are and give them well-paid, interesting (even though dangerous) work in a necessary cause? Everybody knows now why the Fizzle War of 1963 fizzled. The bomb with New York’s number on it didn’t go off, a hundred other things didn’t go as planned—all arranged by the likes of me.
[emphasis mine]

No filme, a existência de uma agência de tempo implica mais de um agente, embora nenhum seja mostrado.

Isso não está claro no filme. Está implícito que, mesmo se houvesse agentes adicionais, pelo menos John é especial, como evidenciado por Robertson:

Understand you are more than an Agent. You're a gift given to the world through a Predestination Paradox. You're the only one free from history, ancestry.

Então John é o único agente em um loop de tempo fechado. Eu suponho que poderia haver outros agentes menos extraordinários, mas nunca vemos um.

No entanto, no conto, All You Zombies , fica claro que existem outros agentes. A resposta de Joe L. fornece algumas evidências bastante strongs , mas se houver alguma dúvida, um dos parágrafos finais do história é bastante explícita que existem muitos recrutas, que definitivamente não são encarnações de John:

I dictated my report; forty recruitments all okayed by the Psych Bureau - counting my own, which I knew would be okayed.

É mencionado por esse barman isso:

I'm a Temporal Agent, one of 11.

De acordo com essa informação, ele não é o único.