How many babies does a gerbil have in a year?

Well, it all depends. If you have a single female brown/tan gerbil (albino included) and one male brown/tan gerbil (albino included) alone in a cage, this answer will apply:

By average, the female will have 5 babies a month. so, if there are 12 months in a year, it will have (by average) 60 babies a year. Don't keep them all, and Don't just let them go. It's against the law to release numerous rodents.