Although there's no set amount of visitors per month that will ensure you can earn a fixed amount of money, it is recommended that you've at least a few hundred visitors per month. As I mentioned previously, there are different monetization strategies that you should follow depending on the number of clicks you get each month.
With this strategy in mind, you will see that even with less than 1000 visits per month, it is entirely possible to generate a profit, however small it may be.