Absolutely, you can carry forward unused visitors from your account quota to the following month.
For example, if you have a quota of 10,000 visitors in your plan and end up using just 7,000 visitors this month, the remaining 3,000 visitors will remain made available to you for the following month. Therefore, the following month’s visitor quota for your account will be 10,000+3,000=13,000 visitors.
However, the maximum visitors in your account cannot be more than double of your existing plan at any given time. For example, if you have a 10,000-visitor plan, your account can have a maximum of 20,000 visitors.