Hi Roli,
I do not recommend an odd number of birds. Regardless of the sex, one bird always gets left out and often gets bullied. If you are getting zebra finches, you might just get 2 or 4 males. If you get females, whether they have nests or not, you will end up with birds laying eggs. And they may of may not start fighting. Usually a group cage is fine as long as you do not give them nests or let them nest in a bowl or the cage floor. But female Zebra finches can be little egg machines, and unless you make changes to try to stop them, some hens will literally lay eggs until she dies from laying too many. Male Zebras tend to get along well – again, no nests, so a cage with males might work out better.
Thank you for asking Lafeber,
Brenda