Ive got 15 in my 30lt cube, but there are other fish in there too. You could easily get 20 in there, if my cube was a species tank, I'd probably have 20-25 as they look great in a shoal.
I've found them to be very hardy, not lost a single one yet. And that includes having a glass intake which I was certain they'd get 'stuck' to due to the power of the flow into glass intakes.
Sam