Within the last year, the 20-year-old has clinched two Commonwealth gold medals, four European titles and three golds at the World Championships in Kazan.
Should Peaty be crowned Olympic champion in Brazil, the breaststroke specialist will be the first British man to win gold in swimming for 28 years.
"If he maintains what he's doing in the 100m, they won't beat him - no chance," Foster told BBC Sport.
"With Adam Peaty and James Guy, and others, I think realistically without getting too over-excited, I could see five or six [Olympic] medals for Britain, with one or two golds."
Last week, GB won nine medals, including five golds, at the Worlds in Russia.