Hi @enwy,
Welcome to the community 
Check this topic created by Yongki : Prevent Double Date Time Reservation and this post : Problem with double reservation - #2 by Yongki
I guess that what you need is a conditional Create reservation action and the condition will be based on the reservations collection count.
I think if you use a drop-down to select the slots this condition will not possible and I think you would need a custom drop-down with a custom list.
Thank you