The co-founder of a children's play centre filmed youngsters undressing after swimming lessons, a court heard.
Former Eddie Catz director Darren Johnson used "peepholes" to film women and girls in toilets and changing rooms at a leisure centre for children.
The number of victims is not known, but a police officer involved in the case estimated it may be as many as 40.
Johnson, 51, from Putney, south-west London, admitted five counts of voyeurism over a 12-year period.
He was caught when two 14-year-old girls spotted his camera-phone from under the cubicle adjacent to them and alerted staff, Kingston Crown Court heard.
After Johnson was identified in CCTV footage, police seized two computers containing hundreds of images of women in various states of undress.
Johnson admitted voyeurism between 1 January 2005 and 5 June 2017.
Two of the charges relate to filming the 14-year-old girls, while a third relates to images of women taken under cubicles at a leisure centre.
Two more charges relate to images of women taken through "peepholes" in toilets and changing rooms at an Eddie Catz soft play centre.
His defence lawyer claimed Johnson had not meant to film teenagers.
However, prosecutor Lauren Sales said CCTV shows Johnson "stalking children around the leisure centre".
"He attended the leisure centre at times when schools have swim classes," she added.
"He goes up to the swimming pool, sees the kids, sees them using the facilities, stalks them to the shower area... and uses the cubicle next to them."
Johnson, who founded Eddie Catz with his wife, resigned from the business as soon as the allegations came to light.
The company has centres in Earlsfield, Putney, Wimbledon and Newbury, as well as a presence in branches of Mothercare in Leeds, Gateshead, Manchester, Solihull and Bristol.
Defence lawyer Tom Williams said Johnson was plagued by alcohol problems and marriage troubles.
"He is truly sorry for what he has done and has done everything he can to address those things in his life which pushed him into an extremely dark place in that period," Mr Williams said.
Johnson has attended Alcoholics Anonymous meetings and classes for sex addicts since his arrest, the court heard.
Judge Andrew Campbell said Johnson would be sentenced on Wednesday.