My daughter adopted a 12 month old male (who was on his own as he had been fighting badly with his brother) and all was well for about three weeks. We then found out that he was a she when she had a litter of 7 babies I could do with some advice on the care of mum as well as advice on the babies.
I've kept a total of 7 degus, but thankfully they were all DEFINITELY boys!
What's happened to you is pretty common though, as degus genitals are a little misleading if the person trying to sex them assumes they are like rats, which they're not.
What's very important is that any boys may be able to impregnate mum pretty early if left with her. Start planning to rehome the boys at 5 or 6 weeks old - they should be in a separate cage at that age at least.
Hopefully you will get a few girls and a few boys. Girls can stay with mum and become a group if you have space.
The forum linked to below is pretty good and there are some very knowledgeable people on there.