How many sales do you need before you start making money?
Break-even is the point where total revenue equals total costs — you're not losing money, but not making it yet. Every sale after break-even is profit. The contribution margin is what each sale contributes toward covering fixed costs after variable costs are paid. Divide fixed costs by contribution margin to get your break-even unit count.
Knowing your break-even is step one. Tracking whether you hit it every month is step two.