She replied: "Just one. He wouldn't tell me the other names." When asked how many there were, she said: "Quite a few."
Miss Flounders, who has not been in court since last Tuesday, told the jury she had initially stuck by Johnson after his arrest because he was "honest" with her and she knew he was "telling the truth".
But, when asked about the future of their relationship today, she said: "We're just going to remain friends."
In two further confirmations of their split, Miss Flounders dramatically admitted to the jury that she saw no future in their relationship.
It was the first time that the trial has heard that Johnson and his girlfriend are no longer an item. The defendant sat in the dock with his head bowed as she made the revelations.
Miss Flounders, who was called as a late witness, had arrived at Bradford Crown Court this morning wearing a black jacket over a light blue top and black jeans.
Almost as soon as her evidence began, Mr Pownall asked Miss Flounders why she decided to stay with the Premier League player after his arrest.
"Because he was honest with me and I knew he was telling the truth," she replied. "We'd just had a daughter. I wanted us to be a family."
In a further question about their relationship, he asked: "Do you see any future in the relationship that you had?". Miss Flounders replied: "Do you mean now? Not at the minute I don't, no."
Miss Flounders then told the jury that, at the time of the incident with the girl in January 2015, she suspected Johnson of being unfaithful, saying their relationship was "not very good". She was pregnant with their little girl at the time.
She admitted she did not have any "hard and fast" evidence. But, during cross-examination by prosecutor Kate Blackwell QC, Miss Flounders said Johnson admitted his infidelities when he returned from the police station and she quizzed him about meeting with the girl.
Miss Blackwell then asked her a series of questions about the details of the encounters between Johnson and the teenager, asking at each stage whether the footballer had mentioned it during that 90-minute conversation.
These details included whether the pair kissed passionately for two minutes, that he caressed her body and that he set up a Snapchat account for the girl.
Other details included that he asked her for naked photos and that he told her he wanted to get her jeans off.
Each time, in a quiet voice, Miss Flounders replied: "No."
However, she did agree that her partner told her he had asked the girl for a thank you kiss.
The court also heard how, as Johnson was arrested over the allegations at his home in Castle Eden, County Durham, he told Miss Flounders that the girl was 16, rather than 15.
It was only two days later that Miss Flounders learned the girl's real age, the court was told.
After he returned from the police station after being questioned, Miss Flounders said she told her partner he was "stupid" and asked why he did it.
"He was actually in shock and he realised what he did. He just said he was being stupid," she told the court.
She also said that Johnson admitted that he "liked" the girl.
Mr Pownall asked Miss Flounders: "Did he ever deny that he kissed passionately a girl under the age of 16?". She replied: "No, he never denied it."
Miss Flounders also told the jury that she recognised the girl's name as one of the fans who would wait for Johnson after the games.
She also said she had been in Twitter contact with the teenager and even wished her happy birthday, just one week or so before Johnson was reported to police.
During her evidence, Miss Flounders was also asked questions about her partner's pubic hair and asked to examine a photograph of his penis.
She said she had never seen his public area shaved. The teenager involved in the incident has told the jury she noticed Johnson was shaved in that area during their encounter.
The witness was also asked about an interview her mother gave to the press after the arrest in which said she Johnson was "100% innocent".
She said the information had not come from her and said: "I didn't agree. I knew he had done something wrong."
She said Johnson told her around September last year that he was going to plead guilty to kissing the 15-year-old. He entered the plea earlier this month and has been accused of delaying the admission so that he could continue playing for Sunderland.
He denies that was the case and said he told the club 'everything' after his arrest.
This afternoon, the jury heard evidence from a phone expert, who analysed messages sent by both Johnson and the girl.
The timing of messages was discussed because the prosecution allege that Johnson was engaged in sexual activity with the girl at the time he said he had already met his teammates.
Miss Blackwell reminded the jury there were 834 messages sent between Johnson and the girl in less than two months, all of which were deleted from Johnson's phone.
The footballer has admitted kissing a 15-year-old girl in his car after grooming the teenager using social media.
But the player denies that more serious sex acts took place between him and the girl in his Range Rover on January 30 last year.
Johnson denies two counts of sexual activity with a child. The trial continues.
