Someone can book more than the available quantities.

I rend 6 rooms in a hostel. I put Qty = 6 and in my booking page (http://www.ikariancentre.gr/?page_id=1431) I can see the available rooms correctly. However someone can book for more than the available rooms (when there are 6, someone may book 60!). Is this normal? What do I do wrong?
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