On the bottom of the left sidebar, you can see the information on the number of bookings for the past month. In the Free version, you can create up to 40 bookings per month. To be fair, they are being counted for the last 30 days.
Don't worry that you would miss the overcross of the limit. Next to the numbers, you can see a thermometer, changing color based on the number of bookings. As soon as you get close to 40 bookings, it will turn red (before that it will be orange and green at the very beginning).
At the moment you reach the limit, Reservio will notify you and offer an upgrade to Premium services.
The number of bookings is based on the type of Premium service. It's all up to you, whether 200, 500 bookings are enough for you or you need unlimited bookings.