Hi,
WAAS device can function in Application Acceleration mode or Central Manager mode.
Application Acceleration will perform optimization and Central Manager manges the WAAS devices. CM is the centre point of management where you will manage/configure all other WAAS devices. To configure a WAE as central manager, you need the appropriate license.
For WAAS deployment, you need a minimum of 3 boxes. 2 boxes as application-acceleration (one in remote and one in HQ/DC) and one as CM.
In your case, you may need 2 (in each DC) + 4 (in each remote site) + 1 (Central Manager) = 7 devices.
AFAIK, To position a particular WAE model, you need to consider few factors including WAN bandwidth and Concurrent TCP connections. There are few other factors I am missing here which others can point to.
HTH,
Nagendra