Is "不特定多数"する a slang term that means to date more than 1 person at the same time? For example, Tanaka-san would date person A on Monday, person B on Wednesday, then person C on Friday and Saturday. "田中くんは不特定多数よくやる人だよね。うらやましいなああ。"
This phrase was used (by me at least) a long time ago, and my memory is poor. It seemed to get my intended meaning across... but I don't remember native speakers saying it, yet its a very weird thing to say. I've no idea how I might have "learned" the phrase.