The term "Quantity available" stated on the product description site of dedicated server models refers on the number of available servers in the data center at the moment of your homepage visit.  
Orders of server models ("On hold"), this means orders that have not been confirmed yet, are not included.

An example:
The reference of the server plan description site:

24 hours: 2 units
2 weeks: 10 units
 
means that 2 servers are currently available in the data center for real and can be provided within 24 hours
as long as there are no more orders on hold from other customers before your order.

Furthermore, 10 additional servers can be expected within the next 2 weeks in the data center, that means
they will be reinstallated or get available by contract termination.