I put 7(rosys) in a 2,000 gallon + intex pool about 4 years ago and they spawned like crazy within weeks(had 100's). I also had goldfish and Koi in there.
However, Last year I put 10(rosys) in a 12.5 gallon mini pond by themselves and nothing has happened.
I did have a hiding cave in the intex pool and there's not really any hiding places in the mini pond, so I don't know if that's the difference.