City dominated the Premier League last season as they had the title wrapped up by mid-April and finished the campaign with a record-breaking 100 points.
That led to fans and pundits labelling City as one of the all-time great sides, but Guardiola feels they need a sustained period of success if they are to be legitimately ranked as one of the best-ever Premier League teams.
"We are the best? I'm not sure of that," said Guardiola in an interview with This is Premier League.
"I'm completely with the people who say that to be considered the best team in the history of English football you have to win more. We won one Premier League (title)."
Guardiola is yet to make any signings this summer but City are close to completing a deal to sign Riyad Mahrez from Leicester and are negotiating with Napoli over a fee for defensive midfielder Jorginho.