The Reds will compete in the Champions League for the first time in five years next season, and Rodgers is eyeing more "quality" additions for his side to compete with the best in Europe.
He told reporters: "It's absolutely right to say the squad isn't big enough and needs to be improved. There needs to be an injection of quality at the club this summer.
"It's hard to define numbers but it will have to be something like five or six."
Liverpool are next in action in the Premier League on Monday as they seek their first top-flight title in 24 years.