When your subscription plan renews every month or year, your reference allowance resets back to the starting total. The reference allowance from the previous month/year expires and is replaced by the new total.
For example, Eddy Fix has a Basic Monthly plan, which allows him to Edifix up to 250 references every month. He Edifixes 240 billable references this month. When his plan renews on the anniversary of his sign-up date, his allowance resets back to 250.
If your workload is highly variable over the course of a year, you may want to consider an annual rather than monthly plan. You can also suspend your subscription if you won't be using it for a while, then restart it when you're back in the saddle; unused references won't carry over, but you also won't pay for references you aren't using.